Add encryption policy, split delivery and SecureMailSender

- delivery: PolicyResolver (address > *@domain > default), RecipientKeyResolver
  (discovery + validation), DeliveryPlanner (REQUIRE refusal, SPLIT/FAIL,
  own copy per Bcc with hidden key IDs), MessageAssembler with sender keys
  for readable sent copies, EnvelopeMimeMessage for per-copy SMTP envelopes
- sender: SecureMailSender with BUILT_IN and RELAY implementations and a
  per-recipient SendReport; relay passes the strictest policy as a header hint
- autoconfigure: mode, default-policy, mixed-mode, recipients, sender-keys
  and relay properties; relay sender is not a bean so spring.mail stays intact
- demo: sends through SecureMailSender

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

README.md

@@ -19,12 +19,38 @@ mvn verify
 | `mailsender-pgp-spring-boot-starter` | dependency bundle for applications |
 | `mailsender-demo` | sample app that sends test mails |
 
+## Usage
+
+Add `pro.thinhha.mailsender:mailsender-pgp-spring-boot-starter`, configure `spring.mail.*` as usual, and send
+through `SecureMailSender` instead of `JavaMailSender`:
+
+```java
+SendReport report = secureMailSender.send(message);   // MimeMessage or SimpleMailMessage
+```
+
+Each recipient's key is looked up via WKD (cached) and validated. The message is then sent PGP/MIME-encrypted to
+recipients with a usable key and in plaintext to the rest (logged as a downgrade), or refused with
+`EncryptionPolicyException` if a `REQUIRE` recipient has no key. `SendReport` lists the outcome per recipient.
+
+```yaml
+mail:
+  pgp:
+    mode: BUILT_IN                 # or RELAY: hand plaintext to an encrypting gateway (mail.pgp.relay.*)
+    default-policy: OPPORTUNISTIC  # REQUIRE | OPPORTUNISTIC | PLAINTEXT
+    mixed-mode: SPLIT              # or FAIL
+    recipients:
+      "[*@partner.example]": REQUIRE
+    sender-keys:                   # your own PUBLIC key, so sent copies stay readable
+      "[noreply@example.com]": file:/etc/mail/noreply.pub.asc
+```
+
+Map keys that are addresses need the `[...]` brackets. See [docs/PLAN.md](docs/PLAN.md) for all options.
+
 ## Demo app
 
-The demo app looks up and validates the recipient's key via WKD, then sends `DEMO_COUNT` test mails
-(default: 1 to `test@thinhha.pro`). With a valid key the mail is PGP/MIME-encrypted (RFC 3156); otherwise it is
-sent in plaintext and a warning is logged. The policy layer (REQUIRE / OPPORTUNISTIC / PLAINTEXT, split
-delivery) comes in milestone 5.
+The demo app sends `DEMO_COUNT` test mails (default: 1 to `test@thinhha.pro`) through `SecureMailSender` and logs
+the outcome per recipient: PGP/MIME-encrypted (RFC 3156) when a valid WKD key is found, otherwise plaintext.
+Set `MAIL_PGP_MODE=RELAY` and `RELAY_HOST`/`RELAY_PORT`/`RELAY_USERNAME`/`RELAY_PASSWORD` to send via a gateway.
 
 ```
 mvn -q install -DskipTests

docs/PLAN.md

@@ -117,8 +117,9 @@ mail.pgp:
 ```
 - `RelaySecureMailSender` uses its own `JavaMailSender` pointed at the relay (the app's normal
   `spring.mail.*` sender is untouched).
-- REQUIRE/OPPORTUNISTIC cannot be enforced by the library in this mode; they are passed as a header hint
-  and enforcement is the gateway's responsibility. Documented and logged at startup.
+- REQUIRE/OPPORTUNISTIC cannot be enforced by the library in this mode. With `policy-header` set, the strictest
+  policy among the recipients (`require` > `opportunistic` > `plaintext`) is passed as a header hint; enforcement is
+  the gateway's responsibility. Logged at startup.
 - Encrypt-to-self is the gateway's responsibility in this mode.
 
 ## Configuration (full shape)
@@ -168,10 +169,14 @@ mail.pgp:
    PGPainless and Bouncy Castle can disagree on a shortened expiry; the validator uses the earlier date.
 4. **Encryptor + PGP/MIME** — round-trip decrypt test, structure test, CRLF canonicalization, attachments. ✅
    Verified live: the demo's mail to `test@thinhha.pro` is addressed to its cv25519 subkey `7CEE00AF05A08313`.
-5. **Policy + delivery planner** — table-driven tests over key/no-key × REQUIRE/OPPORTUNISTIC/PLAINTEXT × To/Cc/Bcc;
-   `EnvelopeMimeMessage`.
-6. **SecureMailSender (BUILT_IN + RELAY)**, `SendReport`, sender keys, metrics, autoconfig switch.
+5. **Policy + delivery planner** — `PolicyResolver`, `RecipientKeyResolver`, `DeliveryPlanner`, `MessageAssembler`,
+   `EnvelopeMimeMessage`; tests over key/no-key × REQUIRE/OPPORTUNISTIC/PLAINTEXT × To/Cc/Bcc. ✅
+   All copies of one message share its Date and Message-ID; duplicates get one copy; nobody encryptable → one
+   plaintext copy (no split).
+6. **SecureMailSender (BUILT_IN + RELAY)**, `SendReport`, sender keys, autoconfig switch. ✅
+   GreenMail integration test checks per-mailbox (envelope) delivery, decryption by recipient and sender, REQUIRE
+   refusal with nothing sent, and relay mode with the policy header. Metrics moved to M8.
 7. **Demo + E2E** with GreenMail; manual check against a real WKD domain, decrypt in Thunderbird.
    Demo app ✅ — encrypts when a valid key is found, otherwise sends plaintext (logged). Verified live:
    `test@thinhha.pro` → found via advanced method, fpr `ADA6EFA0CC1630D602889820112495670FF1B4F0`.
-8. **Hardening** — protected headers, actuator endpoint, docs.
+8. **Hardening** — Micrometer metrics, protected headers, actuator endpoint, docs.

mailsender-demo/src/main/java/pro/thinhha/mailsender/demo/TestMailRunner.java

@@ -1,34 +1,22 @@
 package pro.thinhha.mailsender.demo;
 
-import java.io.IOException;
 import java.time.OffsetDateTime;
-import java.util.HexFormat;
-import java.util.List;
-import java.util.Optional;
 
-import org.bouncycastle.openpgp.api.OpenPGPCertificate;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.springframework.boot.ApplicationArguments;
 import org.springframework.boot.ApplicationRunner;
-import org.springframework.mail.javamail.JavaMailSender;
 import org.springframework.mail.javamail.MimeMessageHelper;
 import org.springframework.stereotype.Component;
 
 import jakarta.mail.MessagingException;
 import jakarta.mail.internet.MimeMessage;
-import pro.thinhha.mailsender.pgp.discovery.DiscoveryResult;
-import pro.thinhha.mailsender.pgp.discovery.KeyDiscovery;
-import pro.thinhha.mailsender.pgp.mime.PgpMimeEncryptor;
-import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
-import pro.thinhha.mailsender.pgp.validation.ValidationResult;
+import pro.thinhha.mailsender.pgp.sender.SecureMailSender;
+import pro.thinhha.mailsender.pgp.sender.SendReport;
 
 /**
- * Looks up and validates the recipient's key, then sends {@code demo.count} test mails: PGP/MIME-encrypted when a
- * valid key was found, otherwise in plaintext (logged as a downgrade).
- * <p>
- * This calls the building blocks directly; the policy layer and {@code SecureMailSender} (milestones 5–6) will
- * replace this wiring.
+ * Sends {@code demo.count} test mails through {@link SecureMailSender}, which encrypts per recipient according to
+ * {@code mail.pgp.*} (or hands them to the relay in RELAY mode), and logs what happened to each recipient.
  */
 @Component
 class TestMailRunner implements ApplicationRunner {
@@ -36,88 +24,35 @@ class TestMailRunner implements ApplicationRunner {
     private static final Logger log = LoggerFactory.getLogger(TestMailRunner.class);
 
     private final DemoProperties properties;
-    private final KeyDiscovery keyDiscovery;
-    private final CertificateValidator validator;
-    private final PgpMimeEncryptor mimeEncryptor;
-    private final JavaMailSender mailSender;
+    private final SecureMailSender mailSender;
 
-    TestMailRunner(DemoProperties properties, KeyDiscovery keyDiscovery, CertificateValidator validator,
-            PgpMimeEncryptor mimeEncryptor, JavaMailSender mailSender) {
+    TestMailRunner(DemoProperties properties, SecureMailSender mailSender) {
         this.properties = properties;
-        this.keyDiscovery = keyDiscovery;
-        this.validator = validator;
-        this.mimeEncryptor = mimeEncryptor;
         this.mailSender = mailSender;
     }
 
     @Override
-    public void run(ApplicationArguments args) throws MessagingException, IOException {
-        String to = properties.to();
-        DiscoveryResult discovery = keyDiscovery.discover(to);
-        Optional<OpenPGPCertificate> key = usableKey(discovery, to);
-
+    public void run(ApplicationArguments args) throws MessagingException {
         for (int i = 1; i <= properties.count(); i++) {
-            MimeMessage message = compose(i, discovery, key.isPresent());
-            if (key.isPresent()) {
-                message = mimeEncryptor.encrypt(message, List.of(key.get()), List.of(), mailSender::createMimeMessage);
-                log.info("Sending test mail {}/{} to {} ENCRYPTED to {}", i, properties.count(), to,
-                        fingerprint(key.get()));
-            } else {
-                log.warn("Sending test mail {}/{} to {} in PLAINTEXT: no usable key", i, properties.count(), to);
-            }
-            mailSender.send(message);
-        }
-    }
-
-    private Optional<OpenPGPCertificate> usableKey(DiscoveryResult discovery, String to) {
-        switch (discovery) {
-            case DiscoveryResult.Found found -> {
-                for (OpenPGPCertificate certificate : found.certificates()) {
-                    ValidationResult result = validator.validate(certificate, to);
-                    switch (result) {
-                        case ValidationResult.Valid valid -> {
-                            log.info("Key for {} found via {}: {}", to, found.source(), fingerprint(certificate));
-                            return Optional.of(valid.certificate());
-                        }
-                        case ValidationResult.Rejected rejected -> log.warn("Key {} for {} rejected: {} ({})",
-                                fingerprint(certificate), to, rejected.reason(), rejected.detail());
-                    }
-                }
-            }
-            case DiscoveryResult.NotFound ignored -> log.info("No key published for {}", to);
-            case DiscoveryResult.Unreachable unreachable ->
-                    log.warn("Key lookup for {} failed: {}", to, unreachable.reason());
-        }
-        return Optional.empty();
-    }
-
-    private MimeMessage compose(int index, DiscoveryResult discovery, boolean encrypted) throws MessagingException {
             MimeMessage message = mailSender.createMimeMessage();
             MimeMessageHelper helper = new MimeMessageHelper(message, "UTF-8");
             if (properties.from() != null && !properties.from().isBlank()) {
                 helper.setFrom(properties.from());
             }
             helper.setTo(properties.to());
-        helper.setSubject("mailsender test %d/%d".formatted(index, properties.count()));
+            helper.setSubject("mailsender test %d/%d".formatted(i, properties.count()));
             helper.setText("""
                     Test mail %d of %d from the mailsender demo, sent %s.
 
-                Key discovery for %s: %s
-                This message was sent %s.
-                """.formatted(index, properties.count(), OffsetDateTime.now(), properties.to(),
-                describe(discovery), encrypted ? "PGP/MIME-encrypted" : "in plaintext"));
-        return message;
-    }
+                    If you can read this in a PGP-capable client that shows it as encrypted,
+                    WKD discovery and PGP/MIME encryption worked.
+                    """.formatted(i, properties.count(), OffsetDateTime.now()));
 
-    private static String describe(DiscoveryResult result) {
-        return switch (result) {
-            case DiscoveryResult.Found found -> "found via " + found.source();
-            case DiscoveryResult.NotFound ignored -> "no key published";
-            case DiscoveryResult.Unreachable u -> "lookup failed (" + u.reason() + ")";
-        };
+            SendReport report = mailSender.send(message);
+            for (SendReport.RecipientOutcome outcome : report.recipients()) {
+                log.info("Test mail {}/{} to {}: {}{}", i, properties.count(), outcome.address(), outcome.outcome(),
+                        outcome.detail() == null ? "" : " (" + outcome.detail() + ")");
             }
-
-    private static String fingerprint(OpenPGPCertificate certificate) {
-        return HexFormat.of().withUpperCase().formatHex(certificate.getFingerprint());
+        }
     }
 }

mailsender-demo/src/main/resources/application.yml

@@ -13,6 +13,23 @@ spring:
       mail.smtp.connectiontimeout: 10000
       mail.smtp.timeout: 10000
 
+mail:
+  pgp:
+    mode: ${MAIL_PGP_MODE:BUILT_IN}            # BUILT_IN | RELAY
+    default-policy: ${MAIL_PGP_DEFAULT_POLICY:OPPORTUNISTIC}
+    mixed-mode: SPLIT
+    # recipients:
+    #   "[*@partner.example]": REQUIRE
+    # Readable sent copies: the sender's own PUBLIC key per From address.
+    # sender-keys:
+    #   "[noreply@example.com]": file:/etc/mail/noreply.pub.asc
+    relay:
+      host: ${RELAY_HOST:}
+      port: ${RELAY_PORT:587}
+      username: ${RELAY_USERNAME:}
+      password: ${RELAY_PASSWORD:}
+      policy-header: X-Encryption-Policy
+
 demo:
   to: ${DEMO_TO:test@thinhha.pro}
   from: ${DEMO_FROM:${SMTP_USERNAME:}}

mailsender-demo/src/test/java/pro/thinhha/mailsender/demo/EncryptedTestMailTest.java

@@ -80,6 +80,6 @@ class EncryptedTestMailTest {
             in.transferTo(inner);
         }
         MimeBodyPart entity = new MimeBodyPart(new ByteArrayInputStream(inner.toByteArray()));
-        assertThat(entity.getContent().toString()).contains("found via STATIC", "PGP/MIME-encrypted");
+        assertThat(entity.getContent().toString()).contains("Test mail 1 of 1");
     }
 }

mailsender-demo/src/test/java/pro/thinhha/mailsender/demo/PlaintextTestMailTest.java

@@ -54,6 +54,6 @@ class PlaintextTestMailTest {
         assertThat(received).hasSize(2);
         assertThat(received[0].getAllRecipients()).extracting(Object::toString).containsExactly("test@thinhha.pro");
         assertThat(received[0].getSubject()).isEqualTo("mailsender test 1/2");
-        assertThat(GreenMailUtil.getBody(received[0])).contains("no key published", "in plaintext");
+        assertThat(GreenMailUtil.getBody(received[0])).contains("Test mail 1 of 2");
     }
 }

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/Delivery.java

@@ -0,0 +1,34 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.List;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+import jakarta.mail.internet.InternetAddress;
+
+/**
+ * One copy of the message to send.
+ *
+ * @param envelope         SMTP recipients of this copy (headers still show the original To/Cc)
+ * @param recipientKeys    certificates to encrypt to; empty for a plaintext copy
+ * @param hiddenRecipients true to encrypt without revealing key IDs (Bcc copies)
+ */
+public record Delivery(List<InternetAddress> envelope, List<OpenPGPCertificate> recipientKeys,
+        boolean hiddenRecipients) {
+
+    public Delivery {
+        envelope = List.copyOf(envelope);
+        recipientKeys = List.copyOf(recipientKeys);
+        if (envelope.isEmpty()) {
+            throw new IllegalArgumentException("A delivery needs at least one envelope recipient");
+        }
+    }
+
+    static Delivery plaintext(List<InternetAddress> envelope) {
+        return new Delivery(envelope, List.of(), false);
+    }
+
+    public boolean encrypted() {
+        return !recipientKeys.isEmpty();
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/DeliveryPlan.java

@@ -0,0 +1,12 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.List;
+
+/** The copies to send and the per-recipient decisions behind them. */
+public record DeliveryPlan(List<Delivery> deliveries, List<RecipientDecision> decisions) {
+
+    public DeliveryPlan {
+        deliveries = List.copyOf(deliveries);
+        decisions = List.copyOf(decisions);
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/DeliveryPlanner.java

@@ -0,0 +1,140 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+import jakarta.mail.internet.InternetAddress;
+
+/**
+ * Turns the recipients of one message into deliveries:
+ * <ul>
+ *     <li>A REQUIRE recipient without a usable key → {@link EncryptionPolicyException}, nothing is sent.</li>
+ *     <li>Nobody gets encryption → one plaintext copy to everyone.</li>
+ *     <li>Otherwise one encrypted copy for To/Cc recipients with keys, one plaintext copy for the other To/Cc
+ *     recipients, and an own copy per Bcc recipient (encrypted with hidden key IDs when possible).
+ *     With {@link MixedMode#FAIL}, a mix of encrypted and plaintext recipients is refused instead.</li>
+ * </ul>
+ * An address listed more than once receives one copy, under its first header.
+ */
+public final class DeliveryPlanner {
+
+    private final PolicyResolver policies;
+    private final RecipientKeyResolver keys;
+    private final MixedMode mixedMode;
+
+    public DeliveryPlanner(PolicyResolver policies, RecipientKeyResolver keys, MixedMode mixedMode) {
+        this.policies = Objects.requireNonNull(policies, "policies");
+        this.keys = Objects.requireNonNull(keys, "keys");
+        this.mixedMode = Objects.requireNonNull(mixedMode, "mixedMode");
+    }
+
+    public DeliveryPlan plan(List<InternetAddress> to, List<InternetAddress> cc, List<InternetAddress> bcc) {
+        Map<String, Recipient> recipients = new LinkedHashMap<>();
+        add(recipients, to, RecipientType.TO);
+        add(recipients, cc, RecipientType.CC);
+        add(recipients, bcc, RecipientType.BCC);
+        if (recipients.isEmpty()) {
+            throw new IllegalArgumentException("Message has no recipients");
+        }
+
+        List<String> missingRequired = new ArrayList<>();
+        for (Recipient recipient : recipients.values()) {
+            recipient.policy = policies.resolve(recipient.email());
+            recipient.key = recipient.policy == EncryptionPolicy.PLAINTEXT
+                    ? new RecipientKey.NotLookedUp()
+                    : keys.resolve(recipient.email());
+            if (recipient.policy == EncryptionPolicy.REQUIRE && !recipient.hasKey()) {
+                missingRequired.add(recipient.email());
+            }
+        }
+        if (!missingRequired.isEmpty()) {
+            throw new EncryptionPolicyException("Encryption required but no usable key", missingRequired);
+        }
+
+        List<Recipient> all = List.copyOf(recipients.values());
+        boolean anyEncrypted = all.stream().anyMatch(Recipient::hasKey);
+        List<String> plaintext = all.stream().filter(r -> !r.hasKey()).map(Recipient::email).toList();
+        if (!anyEncrypted) {
+            return new DeliveryPlan(
+                    List.of(Delivery.plaintext(all.stream().map(Recipient::address).toList())),
+                    decisions(all));
+        }
+        if (mixedMode == MixedMode.FAIL && !plaintext.isEmpty()) {
+            throw new EncryptionPolicyException("Mixed encrypted/plaintext recipients not allowed", plaintext);
+        }
+        return new DeliveryPlan(split(all), decisions(all));
+    }
+
+    private static List<Delivery> split(List<Recipient> all) {
+        List<Delivery> deliveries = new ArrayList<>();
+        List<Recipient> visible = all.stream().filter(r -> r.type != RecipientType.BCC).toList();
+
+        List<Recipient> visibleEncrypted = visible.stream().filter(Recipient::hasKey).toList();
+        if (!visibleEncrypted.isEmpty()) {
+            deliveries.add(new Delivery(addresses(visibleEncrypted), certificates(visibleEncrypted), false));
+        }
+        List<Recipient> visiblePlain = visible.stream().filter(r -> !r.hasKey()).toList();
+        if (!visiblePlain.isEmpty()) {
+            deliveries.add(Delivery.plaintext(addresses(visiblePlain)));
+        }
+        for (Recipient bcc : all) {
+            if (bcc.type == RecipientType.BCC) {
+                deliveries.add(bcc.hasKey()
+                        ? new Delivery(List.of(bcc.address), certificates(List.of(bcc)), true)
+                        : Delivery.plaintext(List.of(bcc.address)));
+            }
+        }
+        return deliveries;
+    }
+
+    private static List<RecipientDecision> decisions(List<Recipient> all) {
+        return all.stream()
+                .map(r -> new RecipientDecision(r.email(), r.type, r.policy, r.key, r.hasKey()))
+                .toList();
+    }
+
+    private static List<InternetAddress> addresses(List<Recipient> recipients) {
+        return recipients.stream().map(Recipient::address).toList();
+    }
+
+    private static List<OpenPGPCertificate> certificates(List<Recipient> recipients) {
+        return recipients.stream().map(r -> ((RecipientKey.Usable) r.key).certificate()).toList();
+    }
+
+    private static void add(Map<String, Recipient> recipients, List<InternetAddress> addresses, RecipientType type) {
+        for (InternetAddress address : addresses) {
+            recipients.putIfAbsent(address.getAddress().toLowerCase(Locale.ROOT), new Recipient(address, type));
+        }
+    }
+
+    private static final class Recipient {
+        final InternetAddress address;
+        final RecipientType type;
+        EncryptionPolicy policy;
+        RecipientKey key;
+
+        Recipient(InternetAddress address, RecipientType type) {
+            this.address = address;
+            this.type = type;
+        }
+
+        InternetAddress address() {
+            return address;
+        }
+
+        String email() {
+            return address.getAddress();
+        }
+
+        /** Will this recipient get an encrypted copy? PLAINTEXT recipients never do. */
+        boolean hasKey() {
+            return key instanceof RecipientKey.Usable;
+        }
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/EncryptionPolicy.java

@@ -0,0 +1,11 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+/** What to do for a recipient, depending on whether a usable key exists. */
+public enum EncryptionPolicy {
+    /** Encrypt; refuse to send if no usable key exists. */
+    REQUIRE,
+    /** Encrypt when a usable key exists, otherwise send in plaintext (logged as a downgrade). */
+    OPPORTUNISTIC,
+    /** Always send in plaintext; no key lookup. */
+    PLAINTEXT
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/EncryptionPolicyException.java

@@ -0,0 +1,19 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.List;
+
+/** The message cannot be sent without violating the encryption policy. Nothing was sent. */
+public class EncryptionPolicyException extends RuntimeException {
+
+    private final List<String> addresses;
+
+    public EncryptionPolicyException(String message, List<String> addresses) {
+        super(message + ": " + String.join(", ", addresses));
+        this.addresses = List.copyOf(addresses);
+    }
+
+    /** The recipients that caused the refusal. */
+    public List<String> addresses() {
+        return addresses;
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/EnvelopeMimeMessage.java

@@ -0,0 +1,35 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.List;
+
+import jakarta.mail.Address;
+import jakarta.mail.MessagingException;
+import jakarta.mail.internet.MimeMessage;
+
+/**
+ * A copy of a message whose SMTP envelope is limited to some recipients while the To/Cc headers stay intact.
+ * {@code JavaMailSenderImpl} (like {@code Transport.send}) uses {@link #getAllRecipients()} as the envelope.
+ */
+public final class EnvelopeMimeMessage extends MimeMessage {
+
+    private final Address[] envelope;
+
+    public EnvelopeMimeMessage(MimeMessage source, List<? extends Address> envelope) throws MessagingException {
+        super(source);
+        this.envelope = envelope.toArray(Address[]::new);
+        removeHeader("Bcc");
+    }
+
+    @Override
+    public Address[] getAllRecipients() {
+        return envelope.clone();
+    }
+
+    /** Keeps the source's Message-ID; the default would generate a new one on every save. */
+    @Override
+    protected void updateMessageID() throws MessagingException {
+        if (getHeader("Message-ID") == null) {
+            super.updateMessageID();
+        }
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/MessageAssembler.java

@@ -0,0 +1,66 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Supplier;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+import jakarta.mail.Address;
+import jakarta.mail.MessagingException;
+import jakarta.mail.internet.InternetAddress;
+import jakarta.mail.internet.MimeMessage;
+import pro.thinhha.mailsender.pgp.mime.PgpMimeEncryptor;
+
+/** Builds the concrete messages for a {@link DeliveryPlan}. */
+public final class MessageAssembler {
+
+    private final PgpMimeEncryptor mimeEncryptor;
+    private final SenderKeyProvider senderKeys;
+
+    public MessageAssembler(PgpMimeEncryptor mimeEncryptor, SenderKeyProvider senderKeys) {
+        this.mimeEncryptor = Objects.requireNonNull(mimeEncryptor, "mimeEncryptor");
+        this.senderKeys = Objects.requireNonNull(senderKeys, "senderKeys");
+    }
+
+    /**
+     * @param newMessage creates empty messages bound to the sending session, e.g.
+     *                   {@code JavaMailSender::createMimeMessage}
+     */
+    public List<MimeMessage> assemble(MimeMessage original, DeliveryPlan plan, Supplier<MimeMessage> newMessage)
+            throws MessagingException, IOException {
+        List<OpenPGPCertificate> selfKeys = senderKey(original);
+        // All copies are the same message to their readers: same Date and Message-ID.
+        if (original.getSentDate() == null) {
+            original.setSentDate(new Date());
+        }
+        original.saveChanges();
+        String messageId = original.getMessageID();
+
+        List<MimeMessage> messages = new ArrayList<>();
+        for (Delivery delivery : plan.deliveries()) {
+            MimeMessage message = original;
+            if (delivery.encrypted()) {
+                List<OpenPGPCertificate> visible = new ArrayList<>(selfKeys);
+                List<OpenPGPCertificate> hidden = new ArrayList<>();
+                (delivery.hiddenRecipients() ? hidden : visible).addAll(delivery.recipientKeys());
+                message = mimeEncryptor.encrypt(original, visible, hidden, newMessage);
+            }
+            EnvelopeMimeMessage copy = new EnvelopeMimeMessage(message, delivery.envelope());
+            copy.setHeader("Message-ID", messageId);
+            messages.add(copy);
+        }
+        return messages;
+    }
+
+    private List<OpenPGPCertificate> senderKey(MimeMessage original) throws MessagingException {
+        Address[] from = original.getFrom();
+        if (from == null || from.length == 0 || !(from[0] instanceof InternetAddress sender)) {
+            return List.of();
+        }
+        return senderKeys.keyFor(sender.getAddress()).map(List::of).orElse(List.of());
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/MixedMode.java

@@ -0,0 +1,9 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+/** What to do when some recipients of one message will get it encrypted and others in plaintext. */
+public enum MixedMode {
+    /** Send an encrypted copy to recipients with keys and a plaintext copy to the rest. */
+    SPLIT,
+    /** Refuse to send. */
+    FAIL
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/PolicyResolver.java

@@ -0,0 +1,44 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.HashMap;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+
+/**
+ * Resolves the {@link EncryptionPolicy} for an address: an exact address rule wins over a {@code *@domain} rule,
+ * which wins over the default. Matching ignores case.
+ */
+public final class PolicyResolver {
+
+    private final EncryptionPolicy defaultPolicy;
+    private final Map<String, EncryptionPolicy> rules;
+
+    public PolicyResolver(EncryptionPolicy defaultPolicy, Map<String, EncryptionPolicy> rules) {
+        this.defaultPolicy = Objects.requireNonNull(defaultPolicy, "defaultPolicy");
+        Map<String, EncryptionPolicy> normalized = new HashMap<>();
+        rules.forEach((pattern, policy) -> {
+            String key = normalize(pattern);
+            if (!key.contains("@")) {
+                throw new IllegalArgumentException(
+                        "Policy rule must be an address or *@domain, got: " + pattern);
+            }
+            normalized.put(key, Objects.requireNonNull(policy, pattern));
+        });
+        this.rules = Map.copyOf(normalized);
+    }
+
+    public EncryptionPolicy resolve(String email) {
+        String address = normalize(email);
+        EncryptionPolicy exact = rules.get(address);
+        if (exact != null) {
+            return exact;
+        }
+        String domain = address.substring(address.lastIndexOf('@') + 1);
+        return rules.getOrDefault("*@" + domain, defaultPolicy);
+    }
+
+    private static String normalize(String value) {
+        return value.strip().toLowerCase(Locale.ROOT);
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/RecipientDecision.java

@@ -0,0 +1,24 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.Objects;
+
+/**
+ * How one recipient will receive the message.
+ *
+ * @param encrypted whether the copy this recipient receives is encrypted
+ */
+public record RecipientDecision(String address, RecipientType type, EncryptionPolicy policy, RecipientKey key,
+        boolean encrypted) {
+
+    public RecipientDecision {
+        Objects.requireNonNull(address, "address");
+        Objects.requireNonNull(type, "type");
+        Objects.requireNonNull(policy, "policy");
+        Objects.requireNonNull(key, "key");
+    }
+
+    /** An OPPORTUNISTIC recipient that gets plaintext because no usable key exists. */
+    public boolean isDowngrade() {
+        return !encrypted && policy == EncryptionPolicy.OPPORTUNISTIC;
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/RecipientKey.java

@@ -0,0 +1,37 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.Objects;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+import pro.thinhha.mailsender.pgp.discovery.DiscoverySource;
+
+/** Whether a recipient has a usable (discovered and validated) key. */
+public sealed interface RecipientKey {
+
+    record Usable(OpenPGPCertificate certificate, DiscoverySource source) implements RecipientKey {
+        public Usable {
+            Objects.requireNonNull(certificate, "certificate");
+            Objects.requireNonNull(source, "source");
+        }
+    }
+
+    record Missing(Reason reason, String detail) implements RecipientKey {
+        public Missing {
+            Objects.requireNonNull(reason, "reason");
+        }
+    }
+
+    /** No lookup was made because the recipient's policy is {@link EncryptionPolicy#PLAINTEXT}. */
+    record NotLookedUp() implements RecipientKey {
+    }
+
+    enum Reason {
+        /** No key published. */
+        NOT_FOUND,
+        /** The lookup failed (timeout, server error, malformed response). */
+        UNREACHABLE,
+        /** Keys were found but none passed validation. */
+        REJECTED
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/RecipientKeyResolver.java

@@ -0,0 +1,46 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+import pro.thinhha.mailsender.pgp.discovery.DiscoveryResult;
+import pro.thinhha.mailsender.pgp.discovery.KeyDiscovery;
+import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
+import pro.thinhha.mailsender.pgp.validation.ValidationResult;
+
+/** Discovers and validates a recipient's key; the first certificate that passes validation is used. */
+public final class RecipientKeyResolver {
+
+    private final KeyDiscovery discovery;
+    private final CertificateValidator validator;
+
+    public RecipientKeyResolver(KeyDiscovery discovery, CertificateValidator validator) {
+        this.discovery = Objects.requireNonNull(discovery, "discovery");
+        this.validator = Objects.requireNonNull(validator, "validator");
+    }
+
+    public RecipientKey resolve(String email) {
+        return switch (discovery.discover(email)) {
+            case DiscoveryResult.Found found -> firstValid(found, email);
+            case DiscoveryResult.NotFound ignored -> new RecipientKey.Missing(RecipientKey.Reason.NOT_FOUND, null);
+            case DiscoveryResult.Unreachable u -> new RecipientKey.Missing(RecipientKey.Reason.UNREACHABLE, u.reason());
+        };
+    }
+
+    private RecipientKey firstValid(DiscoveryResult.Found found, String email) {
+        List<String> rejections = new ArrayList<>();
+        for (OpenPGPCertificate certificate : found.certificates()) {
+            switch (validator.validate(certificate, email)) {
+                case ValidationResult.Valid valid -> {
+                    return new RecipientKey.Usable(valid.certificate(), found.source());
+                }
+                case ValidationResult.Rejected rejected ->
+                        rejections.add(rejected.reason() + " (" + rejected.detail() + ")");
+            }
+        }
+        return new RecipientKey.Missing(RecipientKey.Reason.REJECTED, String.join("; ", rejections));
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/RecipientType.java

@@ -0,0 +1,6 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+/** Header a recipient was addressed in. */
+public enum RecipientType {
+    TO, CC, BCC
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/SenderKeyProvider.java

@@ -0,0 +1,20 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.Optional;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+/**
+ * The sender's own public key, added as an extra recipient so the sender can read the sent copy. Applications that
+ * need readable sent copies must provide a key for each From address; without one the copy is unreadable for the
+ * sender.
+ */
+@FunctionalInterface
+public interface SenderKeyProvider {
+
+    Optional<OpenPGPCertificate> keyFor(String fromAddress);
+
+    static SenderKeyProvider none() {
+        return from -> Optional.empty();
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/delivery/StaticSenderKeyProvider.java

@@ -0,0 +1,38 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import java.util.HashMap;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Optional;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
+import pro.thinhha.mailsender.pgp.validation.ValidationResult;
+
+/** Sender keys from configuration, validated when constructed so a bad key fails at startup, not on send. */
+public final class StaticSenderKeyProvider implements SenderKeyProvider {
+
+    private final Map<String, OpenPGPCertificate> keys;
+
+    public StaticSenderKeyProvider(Map<String, OpenPGPCertificate> keys, CertificateValidator validator) {
+        Map<String, OpenPGPCertificate> normalized = new HashMap<>();
+        keys.forEach((address, certificate) -> {
+            if (validator.validate(certificate, address) instanceof ValidationResult.Rejected rejected) {
+                throw new IllegalArgumentException("Sender key for " + address + " is not usable: "
+                        + rejected.reason() + " (" + rejected.detail() + ")");
+            }
+            normalized.put(address.strip().toLowerCase(Locale.ROOT), certificate);
+        });
+        this.keys = Map.copyOf(normalized);
+    }
+
+    @Override
+    public Optional<OpenPGPCertificate> keyFor(String fromAddress) {
+        return Optional.ofNullable(keys.get(fromAddress.strip().toLowerCase(Locale.ROOT)));
+    }
+
+    public boolean isEmpty() {
+        return keys.isEmpty();
+    }
+}

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/delivery/DeliveryPlannerTest.java

@@ -0,0 +1,167 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+
+import jakarta.mail.internet.InternetAddress;
+import pro.thinhha.mailsender.pgp.discovery.DiscoveryResult;
+import pro.thinhha.mailsender.pgp.discovery.DiscoverySource;
+import pro.thinhha.mailsender.pgp.discovery.KeyDiscovery;
+import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
+
+class DeliveryPlannerTest {
+
+    private static final OpenPGPCertificate ALICE = cert("alice@example.org");
+    private static final OpenPGPCertificate BOB = cert("bob@example.org");
+    private static final OpenPGPCertificate DAVE = cert("dave@example.org");
+
+    private final Map<String, DiscoveryResult> published = new HashMap<>(Map.of(
+            "alice@example.org", found(ALICE),
+            "bob@example.org", found(BOB),
+            "dave@example.org", found(DAVE)));
+    private final Map<String, EncryptionPolicy> rules = new HashMap<>();
+    private int lookups;
+
+    @Test
+    void everyoneWithKeysGetsOneEncryptedCopy() {
+        DeliveryPlan plan = planner(MixedMode.SPLIT).plan(list("alice@example.org"), list("bob@example.org"), list());
+
+        assertThat(plan.deliveries()).singleElement().satisfies(d -> {
+            assertThat(d.encrypted()).isTrue();
+            assertThat(d.hiddenRecipients()).isFalse();
+            assertThat(d.envelope()).extracting(InternetAddress::getAddress)
+                    .containsExactly("alice@example.org", "bob@example.org");
+            assertThat(d.recipientKeys()).containsExactly(ALICE, BOB);
+        });
+        assertThat(plan.decisions()).allMatch(RecipientDecision::encrypted);
+    }
+
+    @Test
+    void nobodyWithKeysGetsOnePlaintextCopyIncludingBcc() {
+        DeliveryPlan plan = planner(MixedMode.FAIL)
+                .plan(list("carol@example.org"), list(), list("erin@example.org"));
+
+        assertThat(plan.deliveries()).singleElement().satisfies(d -> {
+            assertThat(d.encrypted()).isFalse();
+            assertThat(d.envelope()).extracting(InternetAddress::getAddress)
+                    .containsExactly("carol@example.org", "erin@example.org");
+        });
+        assertThat(plan.decisions()).allMatch(RecipientDecision::isDowngrade);
+    }
+
+    @Test
+    void mixedRecipientsAreSplit() {
+        DeliveryPlan plan = planner(MixedMode.SPLIT).plan(
+                list("alice@example.org", "carol@example.org"), list(), list("dave@example.org", "erin@example.org"));
+
+        assertThat(plan.deliveries()).hasSize(4);
+        Delivery encrypted = plan.deliveries().get(0);
+        assertThat(encrypted.encrypted()).isTrue();
+        assertThat(encrypted.envelope()).extracting(InternetAddress::getAddress).containsExactly("alice@example.org");
+
+        Delivery plain = plan.deliveries().get(1);
+        assertThat(plain.encrypted()).isFalse();
+        assertThat(plain.envelope()).extracting(InternetAddress::getAddress).containsExactly("carol@example.org");
+
+        Delivery bccWithKey = plan.deliveries().get(2);
+        assertThat(bccWithKey.encrypted()).isTrue();
+        assertThat(bccWithKey.hiddenRecipients()).isTrue();
+        assertThat(bccWithKey.recipientKeys()).containsExactly(DAVE);
+        assertThat(bccWithKey.envelope()).extracting(InternetAddress::getAddress).containsExactly("dave@example.org");
+
+        Delivery bccWithoutKey = plan.deliveries().get(3);
+        assertThat(bccWithoutKey.encrypted()).isFalse();
+        assertThat(bccWithoutKey.envelope()).extracting(InternetAddress::getAddress)
+                .containsExactly("erin@example.org");
+    }
+
+    @Test
+    void mixedRecipientsAreRefusedInFailMode() {
+        assertThatThrownBy(() -> planner(MixedMode.FAIL).plan(
+                list("alice@example.org", "carol@example.org"), list(), list()))
+                .isInstanceOfSatisfying(EncryptionPolicyException.class,
+                        e -> assertThat(e.addresses()).containsExactly("carol@example.org"));
+    }
+
+    @Test
+    void requiredRecipientWithoutKeyRefusesWholeMessage() {
+        rules.put("*@example.org", EncryptionPolicy.REQUIRE);
+
+        assertThatThrownBy(() -> planner(MixedMode.SPLIT).plan(
+                list("alice@example.org", "carol@example.org"), list(), list()))
+                .isInstanceOfSatisfying(EncryptionPolicyException.class,
+                        e -> assertThat(e.addresses()).containsExactly("carol@example.org"));
+    }
+
+    @Test
+    void plaintextPolicySkipsLookupAndGetsPlaintextCopy() {
+        rules.put("bob@example.org", EncryptionPolicy.PLAINTEXT);
+
+        DeliveryPlan plan = planner(MixedMode.SPLIT).plan(list("alice@example.org", "bob@example.org"), list(), list());
+
+        assertThat(lookups).isEqualTo(1);
+        assertThat(plan.deliveries()).hasSize(2);
+        RecipientDecision bob = plan.decisions().get(1);
+        assertThat(bob.key()).isInstanceOf(RecipientKey.NotLookedUp.class);
+        assertThat(bob.encrypted()).isFalse();
+        assertThat(bob.isDowngrade()).isFalse();
+    }
+
+    @Test
+    void invalidKeyCountsAsMissing() {
+        published.put("carol@example.org", found(cert("mallory@example.org")));
+
+        DeliveryPlan plan = planner(MixedMode.SPLIT).plan(list("carol@example.org"), list(), list());
+
+        assertThat(plan.decisions()).singleElement().satisfies(d -> {
+            assertThat(d.encrypted()).isFalse();
+            assertThat(d.key()).isInstanceOfSatisfying(RecipientKey.Missing.class,
+                    m -> assertThat(m.reason()).isEqualTo(RecipientKey.Reason.REJECTED));
+        });
+    }
+
+    @Test
+    void duplicateAddressGetsOneCopy() {
+        DeliveryPlan plan = planner(MixedMode.SPLIT)
+                .plan(list("alice@example.org"), list("Alice@Example.org"), list("alice@example.org"));
+
+        assertThat(plan.decisions()).singleElement().extracting(RecipientDecision::type).isEqualTo(RecipientType.TO);
+        assertThat(plan.deliveries()).singleElement()
+                .satisfies(d -> assertThat(d.envelope()).hasSize(1));
+    }
+
+    private DeliveryPlanner planner(MixedMode mode) {
+        KeyDiscovery discovery = email -> {
+            lookups++;
+            return published.getOrDefault(email.toLowerCase(), DiscoveryResult.notFound());
+        };
+        return new DeliveryPlanner(new PolicyResolver(EncryptionPolicy.OPPORTUNISTIC, rules),
+                new RecipientKeyResolver(discovery, new CertificateValidator()), mode);
+    }
+
+    private static List<InternetAddress> list(String... addresses) {
+        return java.util.Arrays.stream(addresses).map(a -> {
+            try {
+                return new InternetAddress(a);
+            } catch (jakarta.mail.internet.AddressException e) {
+                throw new IllegalArgumentException(e);
+            }
+        }).toList();
+    }
+
+    private static DiscoveryResult found(OpenPGPCertificate certificate) {
+        return new DiscoveryResult.Found(List.of(certificate), DiscoverySource.WKD_ADVANCED);
+    }
+
+    private static OpenPGPCertificate cert(String email) {
+        return PGPainless.getInstance().generateKey().modernKeyRing("<" + email + ">").toCertificate();
+    }
+}

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/delivery/MessageAssemblerTest.java

@@ -0,0 +1,93 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Properties;
+
+import org.bouncycastle.openpgp.api.OpenPGPKey;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+
+import jakarta.mail.Message;
+import jakarta.mail.Multipart;
+import jakarta.mail.Session;
+import jakarta.mail.internet.InternetAddress;
+import jakarta.mail.internet.MimeMessage;
+import pro.thinhha.mailsender.pgp.crypto.PgpEncryptor;
+import pro.thinhha.mailsender.pgp.crypto.PgpEncryptorTest;
+import pro.thinhha.mailsender.pgp.mime.PgpMimeEncryptor;
+import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
+
+class MessageAssemblerTest {
+
+    private static final PGPainless API = PGPainless.getInstance();
+    private static final OpenPGPKey ALICE = API.generateKey().modernKeyRing("<alice@example.org>");
+    private static final OpenPGPKey DAVE = API.generateKey().modernKeyRing("<dave@example.org>");
+    private static final OpenPGPKey SENDER = API.generateKey().modernKeyRing("<sender@example.com>");
+    private static final Session SESSION = Session.getInstance(new Properties());
+
+    @Test
+    void buildsEnvelopeLimitedCopiesThatShareHeaders() throws Exception {
+        MimeMessage original = new MimeMessage(SESSION);
+        original.setFrom("sender@example.com");
+        original.setRecipients(Message.RecipientType.TO, "alice@example.org, carol@example.org");
+        original.setRecipients(Message.RecipientType.BCC, "dave@example.org");
+        original.setSubject("Split");
+        original.setText("secret");
+
+        DeliveryPlan plan = new DeliveryPlan(List.of(
+                new Delivery(List.of(new InternetAddress("alice@example.org")), List.of(ALICE.toCertificate()), false),
+                Delivery.plaintext(List.of(new InternetAddress("carol@example.org"))),
+                new Delivery(List.of(new InternetAddress("dave@example.org")), List.of(DAVE.toCertificate()), true)),
+                List.of());
+        SenderKeyProvider senderKeys = new StaticSenderKeyProvider(
+                Map.of("Sender@Example.com", SENDER.toCertificate()), new CertificateValidator());
+
+        List<MimeMessage> messages = new MessageAssembler(new PgpMimeEncryptor(new PgpEncryptor()), senderKeys)
+                .assemble(original, plan, () -> new MimeMessage(SESSION));
+
+        assertThat(messages).hasSize(3);
+        assertThat(messages).extracting(MimeMessage::getMessageID).doesNotContainNull().containsOnly(
+                messages.get(0).getMessageID());
+        for (MimeMessage message : messages) {
+            assertThat(message.getHeader("Bcc")).isNull();
+            assertThat(message.getRecipients(Message.RecipientType.TO)).hasSize(2);
+            assertThat(message.getSubject()).isEqualTo("Split");
+        }
+
+        MimeMessage toAlice = messages.get(0);
+        assertThat(toAlice.getAllRecipients()).extracting(Object::toString).containsExactly("alice@example.org");
+        assertThat(toAlice.isMimeType("multipart/encrypted")).isTrue();
+        byte[] armored = encryptedPart(toAlice);
+        assertThat(new String(PgpEncryptorTest.decrypt(armored, ALICE))).contains("secret");
+        assertThat(new String(PgpEncryptorTest.decrypt(armored, SENDER))).as("readable sent copy").contains("secret");
+
+        MimeMessage toCarol = messages.get(1);
+        assertThat(toCarol.getAllRecipients()).extracting(Object::toString).containsExactly("carol@example.org");
+        assertThat(toCarol.getContent()).isEqualTo("secret");
+
+        MimeMessage toDave = messages.get(2);
+        assertThat(toDave.getAllRecipients()).extracting(Object::toString).containsExactly("dave@example.org");
+        assertThat(new String(PgpEncryptorTest.decrypt(encryptedPart(toDave), DAVE))).contains("secret");
+    }
+
+    @Test
+    void staticSenderKeyProviderRejectsUnusableKey() {
+        org.assertj.core.api.Assertions.assertThatIllegalArgumentException().isThrownBy(() ->
+                new StaticSenderKeyProvider(Map.of("other@example.com", SENDER.toCertificate()),
+                        new CertificateValidator()));
+        assertThat(SenderKeyProvider.none().keyFor("x@y.z")).isEqualTo(Optional.empty());
+    }
+
+    private static byte[] encryptedPart(MimeMessage message) throws Exception {
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        message.writeTo(out);
+        MimeMessage parsed = new MimeMessage(SESSION, new ByteArrayInputStream(out.toByteArray()));
+        return ((Multipart) parsed.getContent()).getBodyPart(1).getInputStream().readAllBytes();
+    }
+}

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/delivery/PolicyResolverTest.java

@@ -0,0 +1,35 @@
+package pro.thinhha.mailsender.pgp.delivery;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+class PolicyResolverTest {
+
+    private final PolicyResolver resolver = new PolicyResolver(EncryptionPolicy.OPPORTUNISTIC, Map.of(
+            "*@partner.com", EncryptionPolicy.REQUIRE,
+            "Press@Partner.com", EncryptionPolicy.PLAINTEXT,
+            "*@newsletter.example", EncryptionPolicy.PLAINTEXT));
+
+    @Test
+    void exactAddressWinsOverDomainWinsOverDefault() {
+        assertThat(resolver.resolve("press@partner.com")).isEqualTo(EncryptionPolicy.PLAINTEXT);
+        assertThat(resolver.resolve("CEO@PARTNER.COM")).isEqualTo(EncryptionPolicy.REQUIRE);
+        assertThat(resolver.resolve("someone@newsletter.example")).isEqualTo(EncryptionPolicy.PLAINTEXT);
+        assertThat(resolver.resolve("alice@example.org")).isEqualTo(EncryptionPolicy.OPPORTUNISTIC);
+    }
+
+    @Test
+    void domainRuleDoesNotMatchSubdomains() {
+        assertThat(resolver.resolve("a@sub.partner.com")).isEqualTo(EncryptionPolicy.OPPORTUNISTIC);
+    }
+
+    @Test
+    void rejectsRuleWithoutAt() {
+        assertThatIllegalArgumentException().isThrownBy(() ->
+                new PolicyResolver(EncryptionPolicy.OPPORTUNISTIC, Map.of("partner.com", EncryptionPolicy.REQUIRE)));
+    }
+}

mailsender-pgp-spring-boot-autoconfigure/pom.xml

@@ -21,6 +21,11 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-autoconfigure</artifactId>
         </dependency>
+        <dependency>
+            <!-- JavaMailSender -->
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-context-support</artifactId>
+        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-configuration-processor</artifactId>
@@ -32,6 +37,16 @@
             <artifactId>spring-boot-starter-test</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-mail</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.icegreen</groupId>
+            <artifactId>greenmail-junit5</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <build>

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/autoconfigure/PgpMailAutoConfiguration.java

@@ -7,8 +7,11 @@ import java.util.ArrayList;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Properties;
 
 import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
@@ -17,8 +20,17 @@ import org.springframework.boot.context.properties.EnableConfigurationProperties
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Primary;
 import org.springframework.core.io.Resource;
+import org.springframework.mail.javamail.JavaMailSender;
+import org.springframework.mail.javamail.JavaMailSenderImpl;
+import org.springframework.util.StringUtils;
 
 import pro.thinhha.mailsender.pgp.crypto.PgpEncryptor;
+import pro.thinhha.mailsender.pgp.delivery.DeliveryPlanner;
+import pro.thinhha.mailsender.pgp.delivery.MessageAssembler;
+import pro.thinhha.mailsender.pgp.delivery.PolicyResolver;
+import pro.thinhha.mailsender.pgp.delivery.RecipientKeyResolver;
+import pro.thinhha.mailsender.pgp.delivery.SenderKeyProvider;
+import pro.thinhha.mailsender.pgp.delivery.StaticSenderKeyProvider;
 import pro.thinhha.mailsender.pgp.discovery.CachingKeyDiscovery;
 import pro.thinhha.mailsender.pgp.discovery.Certificates;
 import pro.thinhha.mailsender.pgp.discovery.CompositeKeyDiscovery;
@@ -29,24 +41,28 @@ import pro.thinhha.mailsender.pgp.discovery.StaticKeyDiscovery;
 import pro.thinhha.mailsender.pgp.discovery.WkdFetcher;
 import pro.thinhha.mailsender.pgp.discovery.WkdKeyDiscovery;
 import pro.thinhha.mailsender.pgp.mime.PgpMimeEncryptor;
+import pro.thinhha.mailsender.pgp.sender.BuiltInSecureMailSender;
+import pro.thinhha.mailsender.pgp.sender.RelaySecureMailSender;
+import pro.thinhha.mailsender.pgp.sender.SecureMailSender;
 import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
 
 /**
- * Key discovery (pinned keys first, then WKD behind a cache), certificate validation and PGP/MIME encryption beans.
- * The primary {@link KeyDiscovery} bean is the combined chain; {@link CachingKeyDiscovery} is exposed separately so
- * applications can evict entries.
+ * Wires {@link SecureMailSender} and its parts: key discovery (pinned keys first, then WKD behind a cache),
+ * validation, policy, delivery planning and PGP/MIME encryption. Every bean backs off if the application defines
+ * its own. The primary {@link KeyDiscovery} bean is the combined chain; {@link CachingKeyDiscovery} is exposed
+ * separately so applications can evict entries.
  */
-@AutoConfiguration
+@AutoConfiguration(afterName = "org.springframework.boot.mail.autoconfigure.MailSenderAutoConfiguration")
 @EnableConfigurationProperties(PgpMailProperties.class)
 @ConditionalOnProperty(prefix = "mail.pgp", name = "enabled", havingValue = "true", matchIfMissing = true)
 public class PgpMailAutoConfiguration {
 
+    private static final Logger log = LoggerFactory.getLogger(PgpMailAutoConfiguration.class);
+
     @Bean
     @ConditionalOnMissingBean
     StaticKeyDiscovery pgpStaticKeyDiscovery(PgpMailProperties properties) {
-        Map<String, List<OpenPGPCertificate>> certificates = new LinkedHashMap<>();
-        properties.staticKeys().forEach((email, resource) -> certificates.put(email, read(email, resource)));
-        return new StaticKeyDiscovery(certificates);
+        return new StaticKeyDiscovery(readAll("mail.pgp.static-keys", properties.staticKeys()));
     }
 
     @Bean
@@ -97,15 +113,105 @@ public class PgpMailAutoConfiguration {
         return new PgpMimeEncryptor(encryptor);
     }
 
-    private static List<OpenPGPCertificate> read(String email, Resource resource) {
+    @Bean
+    @ConditionalOnMissingBean
+    PolicyResolver pgpPolicyResolver(PgpMailProperties properties) {
+        return new PolicyResolver(properties.defaultPolicy(), properties.recipients());
+    }
+
+    @Bean
+    @ConditionalOnMissingBean
+    RecipientKeyResolver pgpRecipientKeyResolver(KeyDiscovery discovery, CertificateValidator validator) {
+        return new RecipientKeyResolver(discovery, validator);
+    }
+
+    @Bean
+    @ConditionalOnMissingBean
+    DeliveryPlanner pgpDeliveryPlanner(PgpMailProperties properties, PolicyResolver policies,
+            RecipientKeyResolver keys) {
+        return new DeliveryPlanner(policies, keys, properties.mixedMode());
+    }
+
+    @Bean
+    @ConditionalOnMissingBean
+    SenderKeyProvider pgpSenderKeyProvider(PgpMailProperties properties, CertificateValidator validator) {
+        Map<String, OpenPGPCertificate> keys = new LinkedHashMap<>();
+        readAll("mail.pgp.sender-keys", properties.senderKeys()).forEach((address, certificates) -> {
+            if (certificates.size() != 1) {
+                throw new IllegalStateException(
+                        "mail.pgp.sender-keys[" + address + "] must contain exactly one certificate");
+            }
+            keys.put(address, certificates.getFirst());
+        });
+        if (keys.isEmpty() && properties.mode() == PgpMailProperties.Mode.BUILT_IN) {
+            log.info("No mail.pgp.sender-keys configured: encrypted mail will not be readable by its sender");
+        }
+        return new StaticSenderKeyProvider(keys, validator);
+    }
+
+    @Bean
+    @ConditionalOnMissingBean
+    MessageAssembler pgpMessageAssembler(PgpMimeEncryptor mimeEncryptor, SenderKeyProvider senderKeys) {
+        return new MessageAssembler(mimeEncryptor, senderKeys);
+    }
+
+    @Bean
+    @ConditionalOnMissingBean
+    SecureMailSender pgpSecureMailSender(PgpMailProperties properties, ObjectProvider<JavaMailSender> mailSender,
+            ObjectProvider<DeliveryPlanner> planner, ObjectProvider<MessageAssembler> assembler,
+            PolicyResolver policies) {
+        return switch (properties.mode()) {
+            case BUILT_IN -> {
+                JavaMailSender delegate = mailSender.getIfAvailable();
+                if (delegate == null) {
+                    throw new IllegalStateException(
+                            "mail.pgp.mode=BUILT_IN needs a JavaMailSender bean; set spring.mail.host");
+                }
+                yield new BuiltInSecureMailSender(delegate, planner.getObject(), assembler.getObject());
+            }
+            case RELAY -> {
+                PgpMailProperties.Relay relay = properties.relay();
+                log.info("mail.pgp.mode=RELAY: key discovery, encryption and policy enforcement are delegated to "
+                        + "the gateway at {}:{}", relay.host(), relay.port());
+                yield new RelaySecureMailSender(relaySender(relay), policies, relay.policyHeader());
+            }
+        };
+    }
+
+    /** Not a bean: a second JavaMailSender would make Boot's own {@code spring.mail} sender back off. */
+    private static JavaMailSenderImpl relaySender(PgpMailProperties.Relay relay) {
+        if (!StringUtils.hasText(relay.host())) {
+            throw new IllegalStateException("mail.pgp.mode=RELAY requires mail.pgp.relay.host");
+        }
+        JavaMailSenderImpl sender = new JavaMailSenderImpl();
+        sender.setHost(relay.host());
+        sender.setPort(relay.port());
+        sender.setUsername(relay.username());
+        sender.setPassword(relay.password());
+        Properties javaMail = new Properties();
+        javaMail.setProperty("mail.smtp.auth", String.valueOf(StringUtils.hasText(relay.username())));
+        javaMail.setProperty("mail.smtp.starttls.enable", String.valueOf(relay.starttls()));
+        javaMail.setProperty("mail.smtp.starttls.required", String.valueOf(relay.starttls()));
+        javaMail.putAll(relay.properties());
+        sender.setJavaMailProperties(javaMail);
+        return sender;
+    }
+
+    private static Map<String, List<OpenPGPCertificate>> readAll(String property, Map<String, Resource> resources) {
+        Map<String, List<OpenPGPCertificate>> certificates = new LinkedHashMap<>();
+        resources.forEach((email, resource) -> certificates.put(email, read(property, email, resource)));
+        return certificates;
+    }
+
+    private static List<OpenPGPCertificate> read(String property, String email, Resource resource) {
         try (InputStream in = resource.getInputStream()) {
             List<OpenPGPCertificate> certificates = Certificates.parse(in.readAllBytes());
             if (certificates.isEmpty()) {
-                throw new IllegalStateException("mail.pgp.static-keys[" + email + "]: no certificate in " + resource);
+                throw new IllegalStateException(property + "[" + email + "]: no certificate in " + resource);
             }
             return certificates;
         } catch (IOException e) {
-            throw new UncheckedIOException("mail.pgp.static-keys[" + email + "]: cannot read " + resource, e);
+            throw new UncheckedIOException(property + "[" + email + "]: cannot read " + resource, e);
         }
     }
 }

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/autoconfigure/PgpMailProperties.java

@@ -8,6 +8,8 @@ import org.springframework.boot.context.properties.bind.DefaultValue;
 import org.springframework.core.io.Resource;
 import org.springframework.util.unit.DataSize;
 
+import pro.thinhha.mailsender.pgp.delivery.EncryptionPolicy;
+import pro.thinhha.mailsender.pgp.delivery.MixedMode;
 import pro.thinhha.mailsender.pgp.discovery.WkdFallback;
 
 /**
@@ -17,17 +19,37 @@ import pro.thinhha.mailsender.pgp.discovery.WkdFallback;
  * characters, e.g. {@code mail.pgp.static-keys[bob@legacy.org]=file:/etc/keys/bob.asc}.
  *
  * @param enabled       master switch
- * @param staticKeys pinned certificates per address, checked before WKD
+ * @param mode          encrypt in this application or hand mail to an encrypting relay
+ * @param defaultPolicy policy for recipients without a rule
+ * @param mixedMode     what to do when only some recipients can get encrypted mail
+ * @param recipients    policy rules per address or {@code *@domain}
+ * @param staticKeys    pinned recipient certificates per address, checked before WKD
+ * @param senderKeys    the sender's own public key per From address, so sent copies stay readable
  */
 @ConfigurationProperties("mail.pgp")
 public record PgpMailProperties(
         @DefaultValue("true") boolean enabled,
+        @DefaultValue("BUILT_IN") Mode mode,
+        @DefaultValue("OPPORTUNISTIC") EncryptionPolicy defaultPolicy,
+        @DefaultValue("SPLIT") MixedMode mixedMode,
+        Map<String, EncryptionPolicy> recipients,
         @DefaultValue Wkd wkd,
         @DefaultValue Cache cache,
-        Map<String, Resource> staticKeys) {
+        Map<String, Resource> staticKeys,
+        Map<String, Resource> senderKeys,
+        @DefaultValue Relay relay) {
 
     public PgpMailProperties {
+        recipients = recipients == null ? Map.of() : Map.copyOf(recipients);
         staticKeys = staticKeys == null ? Map.of() : Map.copyOf(staticKeys);
+        senderKeys = senderKeys == null ? Map.of() : Map.copyOf(senderKeys);
+    }
+
+    public enum Mode {
+        /** Discover keys and encrypt in this application, send through the application's {@code JavaMailSender}. */
+        BUILT_IN,
+        /** Send plaintext to an encrypting gateway configured under {@code mail.pgp.relay}. */
+        RELAY
     }
 
     public record Wkd(
@@ -49,4 +71,25 @@ public record PgpMailProperties(
             @DefaultValue("5m") Duration errorTtl,
             @DefaultValue("10000") long maxSize) {
     }
+
+    /**
+     * SMTP connection to the encrypting gateway ({@code mode=RELAY}). The hop carries plaintext, so STARTTLS is
+     * required by default.
+     *
+     * @param policyHeader header that carries the strictest recipient policy as a hint for the gateway; empty for none
+     * @param properties   extra JavaMail properties, e.g. {@code mail.smtp.ssl.trust}
+     */
+    public record Relay(
+            String host,
+            @DefaultValue("587") int port,
+            String username,
+            String password,
+            @DefaultValue("true") boolean starttls,
+            String policyHeader,
+            Map<String, String> properties) {
+
+        public Relay {
+            properties = properties == null ? Map.of() : Map.copyOf(properties);
+        }
+    }
 }

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/sender/AbstractSecureMailSender.java

@@ -0,0 +1,41 @@
+package pro.thinhha.mailsender.pgp.sender;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.springframework.mail.MailParseException;
+import org.springframework.mail.SimpleMailMessage;
+import org.springframework.mail.javamail.MimeMailMessage;
+
+import jakarta.mail.Address;
+import jakarta.mail.Message;
+import jakarta.mail.MessagingException;
+import jakarta.mail.internet.InternetAddress;
+import jakarta.mail.internet.MimeMessage;
+
+abstract class AbstractSecureMailSender implements SecureMailSender {
+
+    @Override
+    public SendReport send(SimpleMailMessage simpleMessage) {
+        MimeMailMessage message = new MimeMailMessage(createMimeMessage());
+        simpleMessage.copyTo(message);
+        return send(message.getMimeMessage());
+    }
+
+    static List<InternetAddress> recipients(MimeMessage message, Message.RecipientType type) {
+        try {
+            Address[] addresses = message.getRecipients(type);
+            List<InternetAddress> result = new ArrayList<>();
+            if (addresses != null) {
+                for (Address address : addresses) {
+                    if (address instanceof InternetAddress internet) {
+                        result.add(internet);
+                    }
+                }
+            }
+            return result;
+        } catch (MessagingException e) {
+            throw new MailParseException("Cannot read " + type + " recipients", e);
+        }
+    }
+}

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/sender/BuiltInSecureMailSender.java

@@ -0,0 +1,76 @@
+package pro.thinhha.mailsender.pgp.sender;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Objects;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.mail.MailPreparationException;
+import org.springframework.mail.javamail.JavaMailSender;
+
+import jakarta.mail.Message;
+import jakarta.mail.MessagingException;
+import jakarta.mail.internet.MimeMessage;
+import pro.thinhha.mailsender.pgp.delivery.DeliveryPlan;
+import pro.thinhha.mailsender.pgp.delivery.DeliveryPlanner;
+import pro.thinhha.mailsender.pgp.delivery.MessageAssembler;
+import pro.thinhha.mailsender.pgp.delivery.RecipientDecision;
+import pro.thinhha.mailsender.pgp.delivery.RecipientKey;
+
+/** Encrypts in this application: plan per recipient, build the copies, send them over one SMTP connection. */
+public final class BuiltInSecureMailSender extends AbstractSecureMailSender {
+
+    private static final Logger log = LoggerFactory.getLogger(BuiltInSecureMailSender.class);
+
+    private final JavaMailSender delegate;
+    private final DeliveryPlanner planner;
+    private final MessageAssembler assembler;
+
+    public BuiltInSecureMailSender(JavaMailSender delegate, DeliveryPlanner planner, MessageAssembler assembler) {
+        this.delegate = Objects.requireNonNull(delegate, "delegate");
+        this.planner = Objects.requireNonNull(planner, "planner");
+        this.assembler = Objects.requireNonNull(assembler, "assembler");
+    }
+
+    @Override
+    public MimeMessage createMimeMessage() {
+        return delegate.createMimeMessage();
+    }
+
+    @Override
+    public SendReport send(MimeMessage message) {
+        DeliveryPlan plan = planner.plan(
+                recipients(message, Message.RecipientType.TO),
+                recipients(message, Message.RecipientType.CC),
+                recipients(message, Message.RecipientType.BCC));
+
+        List<MimeMessage> copies;
+        try {
+            copies = assembler.assemble(message, plan, delegate::createMimeMessage);
+        } catch (MessagingException | IOException e) {
+            throw new MailPreparationException("Could not build encrypted message", e);
+        }
+
+        plan.decisions().stream().filter(RecipientDecision::isDowngrade).forEach(d ->
+                log.warn("Sending to {} in PLAINTEXT (policy {}): {}", d.address(), d.policy(), describe(d.key())));
+        delegate.send(copies.toArray(MimeMessage[]::new));
+
+        SendReport report = new SendReport(plan.decisions().stream()
+                .map(d -> new SendReport.RecipientOutcome(d.address(), d.type(), d.policy(),
+                        d.encrypted() ? SendReport.Outcome.ENCRYPTED : SendReport.Outcome.PLAINTEXT,
+                        d.encrypted() ? null : describe(d.key())))
+                .toList());
+        log.debug("Sent {} cop{}: {}", copies.size(), copies.size() == 1 ? "y" : "ies", report);
+        return report;
+    }
+
+    private static String describe(RecipientKey key) {
+        return switch (key) {
+            case RecipientKey.Usable usable -> "key from " + usable.source();
+            case RecipientKey.Missing missing ->
+                    missing.detail() == null ? missing.reason().toString() : missing.reason() + ": " + missing.detail();
+            case RecipientKey.NotLookedUp ignored -> "plaintext policy";
+        };
+    }
+}

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/sender/RelaySecureMailSender.java

@@ -0,0 +1,71 @@
+package pro.thinhha.mailsender.pgp.sender;
+
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import java.util.Locale;
+import java.util.Objects;
+
+import org.springframework.mail.MailPreparationException;
+import org.springframework.mail.javamail.JavaMailSender;
+
+import jakarta.mail.Message;
+import jakarta.mail.MessagingException;
+import jakarta.mail.internet.InternetAddress;
+import jakarta.mail.internet.MimeMessage;
+import pro.thinhha.mailsender.pgp.delivery.EncryptionPolicy;
+import pro.thinhha.mailsender.pgp.delivery.PolicyResolver;
+import pro.thinhha.mailsender.pgp.delivery.RecipientType;
+
+/**
+ * Hands the plaintext message to an encrypting gateway (for example a GnuPG-based filter using
+ * {@code --auto-key-locate wkd}). Key discovery, encryption and policy enforcement happen there; this sender can
+ * only pass the strictest recipient policy along as a header hint.
+ */
+public final class RelaySecureMailSender extends AbstractSecureMailSender {
+
+    private final JavaMailSender relay;
+    private final PolicyResolver policies;
+    private final String policyHeader;
+
+    /** @param policyHeader header carrying the policy hint, or {@code null} for none */
+    public RelaySecureMailSender(JavaMailSender relay, PolicyResolver policies, String policyHeader) {
+        this.relay = Objects.requireNonNull(relay, "relay");
+        this.policies = Objects.requireNonNull(policies, "policies");
+        this.policyHeader = policyHeader == null || policyHeader.isBlank() ? null : policyHeader;
+    }
+
+    @Override
+    public MimeMessage createMimeMessage() {
+        return relay.createMimeMessage();
+    }
+
+    @Override
+    public SendReport send(MimeMessage message) {
+        List<SendReport.RecipientOutcome> outcomes = new ArrayList<>();
+        addOutcomes(outcomes, recipients(message, Message.RecipientType.TO), RecipientType.TO);
+        addOutcomes(outcomes, recipients(message, Message.RecipientType.CC), RecipientType.CC);
+        addOutcomes(outcomes, recipients(message, Message.RecipientType.BCC), RecipientType.BCC);
+
+        if (policyHeader != null && !outcomes.isEmpty()) {
+            // Enum order is strictest first: REQUIRE, OPPORTUNISTIC, PLAINTEXT.
+            EncryptionPolicy strictest = outcomes.stream().map(SendReport.RecipientOutcome::policy)
+                    .min(Comparator.naturalOrder()).orElseThrow();
+            try {
+                message.setHeader(policyHeader, strictest.name().toLowerCase(Locale.ROOT));
+            } catch (MessagingException e) {
+                throw new MailPreparationException("Could not set " + policyHeader, e);
+            }
+        }
+        relay.send(message);
+        return new SendReport(outcomes);
+    }
+
+    private void addOutcomes(List<SendReport.RecipientOutcome> outcomes, List<InternetAddress> addresses,
+            RecipientType type) {
+        for (InternetAddress address : addresses) {
+            outcomes.add(new SendReport.RecipientOutcome(address.getAddress(), type,
+                    policies.resolve(address.getAddress()), SendReport.Outcome.RELAYED, null));
+        }
+    }
+}

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/sender/SecureMailSender.java

@@ -0,0 +1,25 @@
+package pro.thinhha.mailsender.pgp.sender;
+
+import org.springframework.mail.MailException;
+import org.springframework.mail.SimpleMailMessage;
+
+import jakarta.mail.internet.MimeMessage;
+import pro.thinhha.mailsender.pgp.delivery.EncryptionPolicyException;
+
+/**
+ * Sends mail with OpenPGP encryption according to the configured policy. Build the message as you would for
+ * {@code JavaMailSender} (plaintext); the sender decides per recipient whether it goes out encrypted.
+ */
+public interface SecureMailSender {
+
+    MimeMessage createMimeMessage();
+
+    /**
+     * @throws EncryptionPolicyException if the policy forbids sending (nothing is sent)
+     * @throws MailException             on preparation or SMTP failures
+     */
+    SendReport send(MimeMessage message);
+
+    /** @see #send(MimeMessage) */
+    SendReport send(SimpleMailMessage message);
+}

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/sender/SendReport.java

@@ -0,0 +1,34 @@
+package pro.thinhha.mailsender.pgp.sender;
+
+import java.util.List;
+
+import pro.thinhha.mailsender.pgp.delivery.EncryptionPolicy;
+import pro.thinhha.mailsender.pgp.delivery.RecipientType;
+
+/** What happened to each recipient of a sent message. */
+public record SendReport(List<RecipientOutcome> recipients) {
+
+    public SendReport {
+        recipients = List.copyOf(recipients);
+    }
+
+    public boolean allEncrypted() {
+        return recipients.stream().allMatch(r -> r.outcome() == Outcome.ENCRYPTED);
+    }
+
+    /**
+     * @param detail why the recipient got plaintext (missing key reason), or {@code null}
+     */
+    public record RecipientOutcome(String address, RecipientType type, EncryptionPolicy policy, Outcome outcome,
+            String detail) {
+    }
+
+    public enum Outcome {
+        /** Sent PGP/MIME-encrypted by this application. */
+        ENCRYPTED,
+        /** Sent in plaintext by this application. */
+        PLAINTEXT,
+        /** Handed to the encrypting relay, which decides. */
+        RELAYED
+    }
+}

mailsender-pgp-spring-boot-autoconfigure/src/test/java/pro/thinhha/mailsender/pgp/autoconfigure/PgpMailAutoConfigurationTest.java

@@ -11,7 +11,9 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
 import org.pgpainless.PGPainless;
 import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.mail.autoconfigure.MailSenderAutoConfiguration;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.mail.javamail.JavaMailSender;
 
 import pro.thinhha.mailsender.pgp.discovery.CachingKeyDiscovery;
 import pro.thinhha.mailsender.pgp.discovery.DiscoveryResult;
@@ -20,12 +22,16 @@ import pro.thinhha.mailsender.pgp.discovery.KeyDiscovery;
 import pro.thinhha.mailsender.pgp.discovery.WkdFallback;
 import pro.thinhha.mailsender.pgp.discovery.WkdFetcher;
 import pro.thinhha.mailsender.pgp.mime.PgpMimeEncryptor;
+import pro.thinhha.mailsender.pgp.sender.BuiltInSecureMailSender;
+import pro.thinhha.mailsender.pgp.sender.RelaySecureMailSender;
+import pro.thinhha.mailsender.pgp.sender.SecureMailSender;
 import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
 
 class PgpMailAutoConfigurationTest {
 
     private final ApplicationContextRunner runner = new ApplicationContextRunner()
-            .withConfiguration(AutoConfigurations.of(PgpMailAutoConfiguration.class));
+            .withConfiguration(AutoConfigurations.of(MailSenderAutoConfiguration.class, PgpMailAutoConfiguration.class))
+            .withPropertyValues("spring.mail.host=localhost");
 
     @TempDir
     Path tempDir;
@@ -70,6 +76,48 @@ class PgpMailAutoConfigurationTest {
                 .run(context -> assertThat(context).hasFailed());
     }
 
+    @Test
+    void builtInModeWrapsTheApplicationsMailSender() {
+        runner.run(context -> assertThat(context.getBean(SecureMailSender.class))
+                .isInstanceOf(BuiltInSecureMailSender.class));
+    }
+
+    @Test
+    void builtInModeWithoutMailSenderFailsWithHint() {
+        new ApplicationContextRunner()
+                .withConfiguration(AutoConfigurations.of(PgpMailAutoConfiguration.class))
+                .run(context -> assertThat(context).hasFailed().getFailure()
+                        .rootCause().hasMessageContaining("spring.mail.host"));
+    }
+
+    @Test
+    void relayModeKeepsTheApplicationsMailSender() {
+        runner.withPropertyValues("mail.pgp.mode=relay", "mail.pgp.relay.host=gateway.internal")
+                .run(context -> {
+                    assertThat(context.getBean(SecureMailSender.class)).isInstanceOf(RelaySecureMailSender.class);
+                    assertThat(context).hasSingleBean(JavaMailSender.class);
+                    assertThat(context.getBean(JavaMailSender.class)).extracting("host").isEqualTo("localhost");
+                });
+    }
+
+    @Test
+    void relayModeRequiresHost() {
+        runner.withPropertyValues("mail.pgp.mode=RELAY")
+                .run(context -> assertThat(context).hasFailed().getFailure()
+                        .rootCause().hasMessageContaining("mail.pgp.relay.host"));
+    }
+
+    @Test
+    void senderKeyMustMatchItsAddress() throws IOException {
+        Path key = tempDir.resolve("sender.asc");
+        Files.writeString(key, PGPainless.getInstance().generateKey()
+                .modernKeyRing("<someone-else@example.com>").toCertificate().toAsciiArmoredString());
+
+        runner.withPropertyValues("mail.pgp.sender-keys[noreply@example.com]=file:" + key)
+                .run(context -> assertThat(context).hasFailed().getFailure()
+                        .rootCause().hasMessageContaining("USER_ID_MISMATCH"));
+    }
+
     @Test
     void disabledCreatesNothing() {
         runner.withPropertyValues("mail.pgp.enabled=false")

mailsender-pgp-spring-boot-autoconfigure/src/test/java/pro/thinhha/mailsender/pgp/sender/SecureMailSenderIntegrationTest.java

@@ -0,0 +1,170 @@
+package pro.thinhha.mailsender.pgp.sender;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+
+import org.bouncycastle.openpgp.api.OpenPGPKey;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.pgpainless.PGPainless;
+import org.pgpainless.decryption_verification.ConsumerOptions;
+import org.pgpainless.decryption_verification.DecryptionStream;
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.mail.autoconfigure.MailSenderAutoConfiguration;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.mail.SimpleMailMessage;
+
+import com.icegreen.greenmail.util.GreenMail;
+import com.icegreen.greenmail.util.ServerSetupTest;
+
+import jakarta.mail.Message;
+import jakarta.mail.Multipart;
+import jakarta.mail.internet.MimeMessage;
+import pro.thinhha.mailsender.pgp.autoconfigure.PgpMailAutoConfiguration;
+import pro.thinhha.mailsender.pgp.delivery.EncryptionPolicyException;
+
+/** End to end through the auto-configured {@link SecureMailSender} into GreenMail. WKD is off; keys are pinned. */
+class SecureMailSenderIntegrationTest {
+
+    private static final PGPainless API = PGPainless.getInstance();
+    private static final OpenPGPKey ALICE = API.generateKey().modernKeyRing("<alice@example.org>");
+    private static final OpenPGPKey DAVE = API.generateKey().modernKeyRing("<dave@example.org>");
+    private static final OpenPGPKey SENDER = API.generateKey().modernKeyRing("<app@example.com>");
+
+    @TempDir
+    Path keys;
+    private GreenMail greenMail;
+    private ApplicationContextRunner runner;
+
+    @BeforeEach
+    void setUp() throws Exception {
+        greenMail = new GreenMail(ServerSetupTest.SMTP.dynamicPort());
+        greenMail.start();
+        runner = new ApplicationContextRunner()
+                .withConfiguration(AutoConfigurations.of(MailSenderAutoConfiguration.class,
+                        PgpMailAutoConfiguration.class))
+                .withPropertyValues(
+                        "spring.mail.host=127.0.0.1",
+                        "spring.mail.port=" + greenMail.getSmtp().getPort(),
+                        "mail.pgp.wkd.enabled=false",
+                        "mail.pgp.static-keys[alice@example.org]=file:" + write("alice", ALICE),
+                        "mail.pgp.static-keys[dave@example.org]=file:" + write("dave", DAVE),
+                        "mail.pgp.sender-keys[app@example.com]=file:" + write("app", SENDER),
+                        "mail.pgp.recipients[*@partner.example]=REQUIRE");
+    }
+
+    @AfterEach
+    void tearDown() {
+        greenMail.stop();
+    }
+
+    @Test
+    void splitsMixedRecipientsAndReportsEachOne() {
+        runner.run(context -> {
+            SecureMailSender sender = context.getBean(SecureMailSender.class);
+            SimpleMailMessage message = new SimpleMailMessage();
+            message.setFrom("app@example.com");
+            message.setTo("alice@example.org", "carol@example.org");
+            message.setBcc("dave@example.org");
+            message.setSubject("Report");
+            message.setText("top secret");
+
+            SendReport report = sender.send(message);
+
+            assertThat(report.recipients()).extracting(SendReport.RecipientOutcome::address,
+                    SendReport.RecipientOutcome::outcome).containsExactly(
+                    org.assertj.core.groups.Tuple.tuple("alice@example.org", SendReport.Outcome.ENCRYPTED),
+                    org.assertj.core.groups.Tuple.tuple("carol@example.org", SendReport.Outcome.PLAINTEXT),
+                    org.assertj.core.groups.Tuple.tuple("dave@example.org", SendReport.Outcome.ENCRYPTED));
+            assertThat(report.recipients().get(1).detail()).isEqualTo("NOT_FOUND");
+
+            MimeMessage alice = inbox("alice@example.org");
+            MimeMessage carol = inbox("carol@example.org");
+            MimeMessage dave = inbox("dave@example.org");
+            assertThat(greenMail.getReceivedMessages()).hasSize(3);
+
+            assertThat(alice.isMimeType("multipart/encrypted")).isTrue();
+            assertThat(decrypt(alice, ALICE)).contains("top secret");
+            assertThat(decrypt(alice, SENDER)).as("sender can read sent copy").contains("top secret");
+            assertThat(carol.isMimeType("text/plain")).isTrue();
+            assertThat(carol.getContent().toString()).contains("top secret");
+            assertThat(decrypt(dave, DAVE)).contains("top secret");
+
+            for (MimeMessage received : List.of(alice, carol, dave)) {
+                assertThat(received.getHeader("Bcc")).isNull();
+                assertThat(received.getRecipients(Message.RecipientType.TO)).hasSize(2);
+                assertThat(received.getMessageID()).isEqualTo(alice.getMessageID());
+            }
+        });
+    }
+
+    @Test
+    void refusesRequiredRecipientWithoutKeyAndSendsNothing() {
+        runner.run(context -> {
+            SimpleMailMessage message = new SimpleMailMessage();
+            message.setFrom("app@example.com");
+            message.setTo("alice@example.org", "ceo@partner.example");
+            message.setText("x");
+
+            assertThatThrownBy(() -> context.getBean(SecureMailSender.class).send(message))
+                    .isInstanceOf(EncryptionPolicyException.class).hasMessageContaining("ceo@partner.example");
+            assertThat(greenMail.getReceivedMessages()).isEmpty();
+        });
+    }
+
+    @Test
+    void relayModeSendsPlaintextWithPolicyHint() {
+        runner.withPropertyValues(
+                        "mail.pgp.mode=RELAY",
+                        "mail.pgp.relay.host=127.0.0.1",
+                        "mail.pgp.relay.port=" + greenMail.getSmtp().getPort(),
+                        "mail.pgp.relay.starttls=false",
+                        "mail.pgp.relay.policy-header=X-Encryption-Policy")
+                .run(context -> {
+                    SimpleMailMessage message = new SimpleMailMessage();
+                    message.setFrom("app@example.com");
+                    message.setTo("alice@example.org", "ceo@partner.example");
+                    message.setText("via gateway");
+
+                    SendReport report = context.getBean(SecureMailSender.class).send(message);
+
+                    assertThat(report.recipients()).allMatch(r -> r.outcome() == SendReport.Outcome.RELAYED);
+                    MimeMessage received = greenMail.getReceivedMessages()[0];
+                    assertThat(received.getHeader("X-Encryption-Policy")).containsExactly("require");
+                    assertThat(received.getContent().toString()).contains("via gateway");
+                });
+    }
+
+    private Path write(String name, OpenPGPKey key) throws Exception {
+        Path file = keys.resolve(name + ".asc");
+        Files.writeString(file, key.toCertificate().toAsciiArmoredString());
+        return file;
+    }
+
+    /** The single message delivered to this address's mailbox, i.e. via the SMTP envelope. */
+    private MimeMessage inbox(String address) throws Exception {
+        var user = greenMail.getUserManager().getUserByEmail(address);
+        assertThat(user).as("mailbox for " + address).isNotNull();
+        var stored = greenMail.getManagers().getImapHostManager().getInbox(user).getMessages();
+        assertThat(stored).hasSize(1);
+        return stored.getFirst().getMimeMessage();
+    }
+
+    private static String decrypt(MimeMessage message, OpenPGPKey key) throws Exception {
+        byte[] armored = ((Multipart) message.getContent()).getBodyPart(1).getInputStream().readAllBytes();
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        try (DecryptionStream in = API.processMessage().onInputStream(new ByteArrayInputStream(armored))
+                .withOptions(ConsumerOptions.get(API).addDecryptionKey(key))) {
+            in.transferTo(out);
+        }
+        return out.toString(java.nio.charset.StandardCharsets.UTF_8);
+    }
+}