diff --git a/apps/api/internal/app/app.go b/apps/api/internal/app/app.go
index 5ff360d..2a3524d 100644
--- a/apps/api/internal/app/app.go
+++ b/apps/api/internal/app/app.go
@@ -233,6 +233,13 @@ func (a *App) migrate(ctx context.Context) error {
 		`CREATE INDEX IF NOT EXISTS idx_messages_search ON messages(mailbox_id, subject, from_addr, from_name, snippet)`,
 		`CREATE UNIQUE INDEX IF NOT EXISTS idx_messages_mailbox_raw_path ON messages(mailbox_id, raw_path) WHERE raw_path <> '' AND mailbox_id IS NOT NULL`,
 		`CREATE UNIQUE INDEX IF NOT EXISTS idx_messages_unregistered_raw_path ON messages(raw_path) WHERE raw_path <> '' AND mailbox_id IS NULL`,
+		`CREATE TABLE IF NOT EXISTS sent_message_dedupe_keys (
+			mailbox_id TEXT NOT NULL REFERENCES mailboxes(id) ON DELETE CASCADE,
+			folder_id TEXT NOT NULL REFERENCES folders(id) ON DELETE CASCADE,
+			message_id TEXT NOT NULL,
+			created_at TEXT NOT NULL,
+			PRIMARY KEY(mailbox_id, folder_id, message_id)
+		)`,
 		`CREATE TABLE IF NOT EXISTS attachments (
 			id TEXT PRIMARY KEY,
 			message_id TEXT NOT NULL REFERENCES messages(id) ON DELETE CASCADE,
diff --git a/apps/api/internal/app/app_test.go b/apps/api/internal/app/app_test.go
index ff620ec..ca1dee2 100644
--- a/apps/api/internal/app/app_test.go
+++ b/apps/api/internal/app/app_test.go
@@ -960,6 +960,17 @@ func TestSubmissionSentCopyDedupesByMessageID(t *testing.T) {
 	}
 }
 
+func TestSentMessageDedupeTableExists(t *testing.T) {
+	a := newTestApp(t)
+	var count int
+	if err := a.db.QueryRow(`SELECT COUNT(1) FROM sqlite_master WHERE type='table' AND name='sent_message_dedupe_keys'`).Scan(&count); err != nil {
+		t.Fatal(err)
+	}
+	if count != 1 {
+		t.Fatalf("sent message dedupe table count=%d, want 1", count)
+	}
+}
+
 func TestSubmissionServersAcceptStartTLSAndImplicitTLS(t *testing.T) {
 	a := newTestApp(t)
 	host, port, received := startCapturingSMTP(t, 2)
diff --git a/apps/api/internal/app/submission.go b/apps/api/internal/app/submission.go
index ef0e764..e745987 100644
--- a/apps/api/internal/app/submission.go
+++ b/apps/api/internal/app/submission.go
@@ -286,6 +286,9 @@ func (a *App) submitSMTPMessage(ctx context.Context, user *User, mb *Mailbox, ma
 		if err := a.sendSMTP(mb.Address, recipients, prepared); err != nil {
 			if sentID != "" {
 				a.deleteMessage(ctx, sentID)
+				if sentFolderID, ferr := a.ensureFolder(ctx, mb.ID, "Sent"); ferr == nil {
+					a.deleteSentDedupeKey(ctx, mb.ID, sentFolderID, msg.MessageID)
+				}
 			}
 			return smtpError(451, smtpserver.EnhancedCode{4, 4, 0}, "smtp relay failed")
 		}
@@ -363,13 +366,52 @@ func (a *App) insertSentMessageOnce(ctx context.Context, msg storedMessage, atta
 		var existing string
 		err := a.db.QueryRowContext(ctx, `SELECT id FROM messages WHERE mailbox_id=? AND folder_id=? AND message_id=? AND message_id <> '' LIMIT 1`, msg.MailboxID, sentFolderID, msg.MessageID).Scan(&existing)
 		if err == nil {
+			if err := a.insertSentDedupeKey(ctx, msg.MailboxID, sentFolderID, msg.MessageID); err != nil && !errors.Is(err, errSentDedupeExists) {
+				return "", err
+			}
 			return "", nil
 		}
 		if err != nil && !errors.Is(err, sql.ErrNoRows) {
 			return "", err
 		}
+		if err := a.insertSentDedupeKey(ctx, msg.MailboxID, sentFolderID, msg.MessageID); err != nil {
+			if errors.Is(err, errSentDedupeExists) {
+				return "", nil
+			}
+			return "", err
+		}
 	}
-	return a.insertMessage(ctx, msg, attachments)
+	id, err := a.insertMessage(ctx, msg, attachments)
+	if err != nil {
+		if msg.MessageID != "" {
+			a.deleteSentDedupeKey(ctx, msg.MailboxID, sentFolderID, msg.MessageID)
+		}
+		return "", err
+	}
+	return id, nil
+}
+
+var errSentDedupeExists = errors.New("sent message already exists")
+
+func (a *App) insertSentDedupeKey(ctx context.Context, mailboxID, folderID, messageID string) error {
+	if strings.TrimSpace(messageID) == "" {
+		return nil
+	}
+	res, err := a.db.ExecContext(ctx, `INSERT OR IGNORE INTO sent_message_dedupe_keys(mailbox_id,folder_id,message_id,created_at) VALUES(?,?,?,?)`, mailboxID, folderID, messageID, a.now().UTC().Format(time.RFC3339Nano))
+	if err != nil {
+		return err
+	}
+	if rows, err := res.RowsAffected(); err == nil && rows == 0 {
+		return errSentDedupeExists
+	}
+	return nil
+}
+
+func (a *App) deleteSentDedupeKey(ctx context.Context, mailboxID, folderID, messageID string) {
+	if strings.TrimSpace(messageID) == "" {
+		return
+	}
+	_, _ = a.db.ExecContext(ctx, `DELETE FROM sent_message_dedupe_keys WHERE mailbox_id=? AND folder_id=? AND message_id=?`, mailboxID, folderID, messageID)
 }
 
 func readMessageHeader(raw []byte) (textproto.MIMEHeader, []byte, error) {
diff --git a/deploy/.env.example b/deploy/.env.example
index 161928d..9535933 100644
--- a/deploy/.env.example
+++ b/deploy/.env.example
@@ -84,9 +84,13 @@ LANQIN_TURNSTILE_SECRET_KEY=
 # SMTP 发信
 # =========================
 # 单容器部署默认提交给容器内 Postfix。
+# Split stack 会由 docker-compose.stack.yml 默认覆盖为 postfix:25。
+# 如需在 split stack 使用外部 SMTP，可设置 LANQIN_STACK_SMTP_HOST / LANQIN_STACK_SMTP_PORT。
 # 如果要走外部 SMTP，把 Host/Port/Username/Password 改成外部服务配置。
 LANQIN_SMTP_HOST=127.0.0.1
 LANQIN_SMTP_PORT=25
+LANQIN_STACK_SMTP_HOST=
+LANQIN_STACK_SMTP_PORT=
 LANQIN_SMTP_USERNAME=
 LANQIN_SMTP_PASSWORD=
 
diff --git a/deploy/README.md b/deploy/README.md
index 08ea93a..c217419 100644
--- a/deploy/README.md
+++ b/deploy/README.md
@@ -170,6 +170,8 @@ LANQIN_SMTP_PORT=25
 LANQIN_SMTP_REQUIRE_TLS=false
 ```
 
+Split stack 使用 `docker-compose.stack.yml` 时，API 容器默认会把 `LANQIN_SMTP_HOST` 覆盖为 `postfix`，让 Webmail 和 SMTP 提交都 relay 到 Postfix service。只有改用外部 SMTP 时才需要在 `.env` 明确填写 `LANQIN_STACK_SMTP_HOST` / `LANQIN_STACK_SMTP_PORT`。
+
 如果页面提示 `smtp delivery failed: EOF`，通常是 Postfix 会话被中断。优先检查：
 
 ```bash
diff --git a/deploy/docker-compose.stack.yml b/deploy/docker-compose.stack.yml
index b11f079..e54624d 100644
--- a/deploy/docker-compose.stack.yml
+++ b/deploy/docker-compose.stack.yml
@@ -2,6 +2,9 @@ services:
   api:
     image: ${LANQIN_API_IMAGE:-ghcr.io/lanqin996/lanqin-email-api:latest}
     env_file: .env
+    environment:
+      LANQIN_SMTP_HOST: ${LANQIN_STACK_SMTP_HOST:-postfix}
+      LANQIN_SMTP_PORT: ${LANQIN_STACK_SMTP_PORT:-25}
     volumes:
       - ./data:/data
       - ./mail:/var/mail/vhosts:ro