fix(email): unify SMTP connection path between send and test-connection

- UseTLS now tries implicit TLS first (port 465 semantics) and, when the
  server answers in plaintext (tls.RecordHeaderError, e.g. port 587
  submission), automatically retries with mandatory STARTTLS; encryption
  is never silently downgraded (fixes #1470, supersedes #1488)
- TestSMTPConnectionWithConfig now shares connectSMTP with the send path,
  adding the opportunistic STARTTLS upgrade the send path gained in
  b402c367d; this removes the 'test connection fails but test email
  sends' mismatch reported in #1488
- test-connection now also honors dial/IO timeouts and ignores
  non-standard QUIT responses, matching the send path

backend/internal/service/email_service.go

@@ -5,7 +5,9 @@ import (
 	"crypto/rand"
 	"crypto/subtle"
 	"crypto/tls"
+	"crypto/x509"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"html"
 	"log/slog"
@@ -191,118 +193,114 @@ func (s *EmailService) SendEmailWithConfig(config *SMTPConfig, to, subject, body
 		return err
 	}
 
-	addr := fmt.Sprintf("%s:%d", config.Host, config.Port)
-	auth := smtp.PlainAuth("", config.Username, config.Password, config.Host)
-
-	if config.UseTLS {
-		return s.sendMailTLS(addr, auth, message.envelopeFrom, message.envelopeTo, message.data, config.Host)
-	}
-
-	return s.sendMailPlain(addr, auth, message.envelopeFrom, message.envelopeTo, message.data, config.Host)
-}
-
-// sendMailPlain sends mail without TLS using a dialer with timeout.
-func (s *EmailService) sendMailPlain(addr string, auth smtp.Auth, from, to string, msg []byte, host string) error {
-	dialer := &net.Dialer{Timeout: smtpDialTimeout}
-	conn, err := dialer.Dial("tcp", addr)
+	client, err := s.connectSMTP(config)
 	if err != nil {
-		return fmt.Errorf("smtp dial: %w", err)
-	}
-	_ = conn.SetDeadline(time.Now().Add(smtpIOTimeout))
-	defer func() { _ = conn.Close() }()
-
-	client, err := smtp.NewClient(conn, host)
-	if err != nil {
-		return fmt.Errorf("new smtp client: %w", err)
+		return err
 	}
 	defer func() { _ = client.Close() }()
 
-	// Opportunistic STARTTLS: upgrade to encrypted connection if the server supports it.
-	// This mirrors the behavior of smtp.SendMail which we replaced for timeout support.
-	if ok, _ := client.Extension("STARTTLS"); ok {
-		if err = client.StartTLS(&tls.Config{ServerName: host, MinVersion: tls.VersionTLS12}); err != nil {
-			return fmt.Errorf("starttls: %w", err)
-		}
-	}
-
+	auth := smtp.PlainAuth("", config.Username, config.Password, config.Host)
 	if err = client.Auth(auth); err != nil {
 		return fmt.Errorf("smtp auth: %w", err)
 	}
-	if err = client.Mail(from); err != nil {
+	if err = client.Mail(message.envelopeFrom); err != nil {
 		return fmt.Errorf("smtp mail: %w", err)
 	}
-	if err = client.Rcpt(to); err != nil {
+	if err = client.Rcpt(message.envelopeTo); err != nil {
 		return fmt.Errorf("smtp rcpt: %w", err)
 	}
 	w, err := client.Data()
 	if err != nil {
 		return fmt.Errorf("smtp data: %w", err)
 	}
-	if _, err = w.Write(msg); err != nil {
+	if _, err = w.Write(message.data); err != nil {
 		return fmt.Errorf("write msg: %w", err)
 	}
 	if err = w.Close(); err != nil {
 		return fmt.Errorf("close writer: %w", err)
 	}
-	_ = client.Quit()
-	return nil
-}
-
-// sendMailTLS 使用TLS发送邮件
-func (s *EmailService) sendMailTLS(addr string, auth smtp.Auth, from, to string, msg []byte, host string) error {
-	tlsConfig := &tls.Config{
-		ServerName: host,
-		// 强制 TLS 1.2+，避免协议降级导致的弱加密风险。
-		MinVersion: tls.VersionTLS12,
-	}
-
-	dialer := &net.Dialer{Timeout: smtpDialTimeout}
-	conn, err := tls.DialWithDialer(dialer, "tcp", addr, tlsConfig)
-	if err != nil {
-		return fmt.Errorf("tls dial: %w", err)
-	}
-	_ = conn.SetDeadline(time.Now().Add(smtpIOTimeout))
-	defer func() { _ = conn.Close() }()
-
-	client, err := smtp.NewClient(conn, host)
-	if err != nil {
-		return fmt.Errorf("new smtp client: %w", err)
-	}
-	defer func() { _ = client.Close() }()
-
-	if err = client.Auth(auth); err != nil {
-		return fmt.Errorf("smtp auth: %w", err)
-	}
-
-	if err = client.Mail(from); err != nil {
-		return fmt.Errorf("smtp mail: %w", err)
-	}
-
-	if err = client.Rcpt(to); err != nil {
-		return fmt.Errorf("smtp rcpt: %w", err)
-	}
-
-	w, err := client.Data()
-	if err != nil {
-		return fmt.Errorf("smtp data: %w", err)
-	}
-
-	_, err = w.Write(msg)
-	if err != nil {
-		return fmt.Errorf("write msg: %w", err)
-	}
-
-	err = w.Close()
-	if err != nil {
-		return fmt.Errorf("close writer: %w", err)
-	}
-
 	// Email is sent successfully after w.Close(), ignore Quit errors
 	// Some SMTP servers return non-standard responses on QUIT
 	_ = client.Quit()
 	return nil
 }
 
+// smtpTestRootCAs 仅供单元测试注入自签 CA，生产环境始终为 nil（走系统信任链）。
+var smtpTestRootCAs *x509.CertPool
+
+func smtpTLSConfig(host string) *tls.Config {
+	return &tls.Config{
+		ServerName: host,
+		// 强制 TLS 1.2+，避免协议降级导致的弱加密风险。
+		MinVersion: tls.VersionTLS12,
+		RootCAs:    smtpTestRootCAs,
+	}
+}
+
+// connectSMTP 按配置建立 SMTP 会话，发送与测试连接共用此路径，
+// 保证"测试连接成功 ⇔ 实际发信可用"：
+//   - UseTLS=true：先尝试隐式 TLS（465 语义）；若服务器以明文应答
+//     （587/25 等提交端口的 STARTTLS 语义），自动改走"明文连接 + 强制 STARTTLS"。
+//     两种方式都无法建立加密连接时报错，绝不明文继续。
+//   - UseTLS=false：明文连接后若服务器支持 STARTTLS 则机会式升级，
+//     与 smtp.SendMail 的默认行为一致。
+func (s *EmailService) connectSMTP(config *SMTPConfig) (*smtp.Client, error) {
+	addr := fmt.Sprintf("%s:%d", config.Host, config.Port)
+	dialer := &net.Dialer{Timeout: smtpDialTimeout}
+	tlsConfig := smtpTLSConfig(config.Host)
+
+	if config.UseTLS {
+		conn, err := tls.DialWithDialer(dialer, "tcp", addr, tlsConfig)
+		if err == nil {
+			return newSMTPClient(conn, config.Host)
+		}
+		var recordErr tls.RecordHeaderError
+		if !errors.As(err, &recordErr) {
+			return nil, fmt.Errorf("tls dial: %w", err)
+		}
+		// SMTP 服务器先发问候语：明文问候会让 TLS 握手立刻返回
+		// RecordHeaderError，据此可靠判定对端期望 STARTTLS。
+		return s.connectSMTPStartTLS(dialer, addr, config.Host, tlsConfig, true)
+	}
+
+	return s.connectSMTPStartTLS(dialer, addr, config.Host, tlsConfig, false)
+}
+
+// connectSMTPStartTLS 建立明文连接并按需升级 STARTTLS。
+// mandatory 为 true 时服务器必须支持 STARTTLS，否则报错。
+func (s *EmailService) connectSMTPStartTLS(dialer *net.Dialer, addr, host string, tlsConfig *tls.Config, mandatory bool) (*smtp.Client, error) {
+	conn, err := dialer.Dial("tcp", addr)
+	if err != nil {
+		return nil, fmt.Errorf("smtp dial: %w", err)
+	}
+	client, err := newSMTPClient(conn, host)
+	if err != nil {
+		return nil, err
+	}
+	if ok, _ := client.Extension("STARTTLS"); !ok {
+		if mandatory {
+			_ = client.Close()
+			return nil, errors.New("smtp server does not support STARTTLS")
+		}
+		return client, nil
+	}
+	if err := client.StartTLS(tlsConfig); err != nil {
+		_ = client.Close()
+		return nil, fmt.Errorf("starttls: %w", err)
+	}
+	return client, nil
+}
+
+func newSMTPClient(conn net.Conn, host string) (*smtp.Client, error) {
+	_ = conn.SetDeadline(time.Now().Add(smtpIOTimeout))
+	client, err := smtp.NewClient(conn, host)
+	if err != nil {
+		_ = conn.Close()
+		return nil, fmt.Errorf("new smtp client: %w", err)
+	}
+	return client, nil
+}
+
 // GenerateVerifyCode 生成6位数字验证码
 func (s *EmailService) GenerateVerifyCode() (string, error) {
 	const digits = "0123456789"
@@ -451,49 +449,24 @@ func (s *EmailService) buildVerifyCodeEmailBody(code, siteName string) string {
 `, html.EscapeString(siteName), code)
 }
 
-// TestSMTPConnectionWithConfig 使用指定配置测试SMTP连接
+// TestSMTPConnectionWithConfig 使用指定配置测试SMTP连接。
+// 与 SendEmailWithConfig 共用 connectSMTP 建连（含 STARTTLS 升级逻辑），
+// 避免出现"测试连接失败但实际发信成功"的不一致。
 func (s *EmailService) TestSMTPConnectionWithConfig(config *SMTPConfig) error {
-	addr := fmt.Sprintf("%s:%d", config.Host, config.Port)
-
-	if config.UseTLS {
-		tlsConfig := &tls.Config{
-			ServerName: config.Host,
-			// 与发送逻辑一致，显式要求 TLS 1.2+。
-			MinVersion: tls.VersionTLS12,
-		}
-		conn, err := tls.Dial("tcp", addr, tlsConfig)
-		if err != nil {
-			return fmt.Errorf("tls connection failed: %w", err)
-		}
-		defer func() { _ = conn.Close() }()
-
-		client, err := smtp.NewClient(conn, config.Host)
-		if err != nil {
-			return fmt.Errorf("smtp client creation failed: %w", err)
-		}
-		defer func() { _ = client.Close() }()
-
-		auth := smtp.PlainAuth("", config.Username, config.Password, config.Host)
-		if err = client.Auth(auth); err != nil {
-			return fmt.Errorf("smtp authentication failed: %w", err)
-		}
-
-		return client.Quit()
-	}
-
-	// 非TLS连接测试
-	client, err := smtp.Dial(addr)
+	client, err := s.connectSMTP(config)
 	if err != nil {
 		return fmt.Errorf("smtp connection failed: %w", err)
 	}
 	defer func() { _ = client.Close() }()
 
 	auth := smtp.PlainAuth("", config.Username, config.Password, config.Host)
-	if err = client.Auth(auth); err != nil {
+	if err := client.Auth(auth); err != nil {
 		return fmt.Errorf("smtp authentication failed: %w", err)
 	}
 
-	return client.Quit()
+	// 认证成功即视为连接可用；与发送路径一致，忽略 QUIT 的非标准响应。
+	_ = client.Quit()
+	return nil
 }
 
 // GeneratePasswordResetToken generates a secure 32-byte random token (64 hex characters)

backend/internal/service/email_service_smtp_test.go

@@ -0,0 +1,383 @@
+//go:build unit
+
+package service
+
+import (
+	"bufio"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"math/big"
+	"net"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// newSMTPTestCert 生成 127.0.0.1/localhost 的自签证书及其信任池。
+func newSMTPTestCert(t *testing.T) (tls.Certificate, *x509.CertPool) {
+	t.Helper()
+	priv, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatalf("generate key: %v", err)
+	}
+	template := x509.Certificate{
+		SerialNumber:          big.NewInt(1),
+		Subject:               pkix.Name{CommonName: "127.0.0.1"},
+		NotBefore:             time.Now().Add(-time.Hour),
+		NotAfter:              time.Now().Add(time.Hour),
+		KeyUsage:              x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign,
+		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+		BasicConstraintsValid: true,
+		IsCA:                  true,
+		IPAddresses:           []net.IP{net.ParseIP("127.0.0.1")},
+		DNSNames:              []string{"localhost"},
+	}
+	der, err := x509.CreateCertificate(rand.Reader, &template, &template, &priv.PublicKey, priv)
+	if err != nil {
+		t.Fatalf("create certificate: %v", err)
+	}
+	leaf, err := x509.ParseCertificate(der)
+	if err != nil {
+		t.Fatalf("parse certificate: %v", err)
+	}
+	pool := x509.NewCertPool()
+	pool.AddCert(leaf)
+	return tls.Certificate{Certificate: [][]byte{der}, PrivateKey: priv}, pool
+}
+
+// fakeSMTPServer 是覆盖三种连接形态的最小 SMTP 服务器：
+// 隐式 TLS（465 语义）、明文+STARTTLS（587 语义）、纯明文。
+type fakeSMTPServer struct {
+	listener          net.Listener
+	tlsConfig         *tls.Config
+	advertiseStartTLS bool
+
+	mu       sync.Mutex
+	commands []string
+	conns    atomic.Int64
+	wg       sync.WaitGroup
+}
+
+func startFakeSMTPServer(t *testing.T, implicitTLS, advertiseStartTLS bool) (*fakeSMTPServer, int) {
+	t.Helper()
+	cert, pool := newSMTPTestCert(t)
+	prevPool := smtpTestRootCAs
+	smtpTestRootCAs = pool
+	t.Cleanup(func() { smtpTestRootCAs = prevPool })
+
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	srv := &fakeSMTPServer{
+		listener:          listener,
+		tlsConfig:         &tls.Config{Certificates: []tls.Certificate{cert}, MinVersion: tls.VersionTLS12},
+		advertiseStartTLS: advertiseStartTLS,
+	}
+	if implicitTLS {
+		srv.listener = tls.NewListener(listener, srv.tlsConfig)
+	}
+	t.Cleanup(func() {
+		_ = srv.listener.Close()
+		srv.wg.Wait()
+	})
+
+	srv.wg.Add(1)
+	go func() {
+		defer srv.wg.Done()
+		for {
+			conn, err := srv.listener.Accept()
+			if err != nil {
+				return
+			}
+			srv.conns.Add(1)
+			srv.wg.Add(1)
+			go func() {
+				defer srv.wg.Done()
+				defer func() { _ = conn.Close() }()
+				_ = conn.SetDeadline(time.Now().Add(10 * time.Second))
+				srv.serve(conn, srv.advertiseStartTLS)
+			}()
+		}
+	}()
+
+	port := listener.Addr().(*net.TCPAddr).Port
+	return srv, port
+}
+
+func (srv *fakeSMTPServer) record(cmd string) {
+	srv.mu.Lock()
+	defer srv.mu.Unlock()
+	srv.commands = append(srv.commands, cmd)
+}
+
+func (srv *fakeSMTPServer) sawCommand(prefix string) bool {
+	srv.mu.Lock()
+	defer srv.mu.Unlock()
+	for _, cmd := range srv.commands {
+		if strings.HasPrefix(strings.ToUpper(cmd), prefix) {
+			return true
+		}
+	}
+	return false
+}
+
+func (srv *fakeSMTPServer) serve(conn net.Conn, allowStartTLS bool) {
+	reader := bufio.NewReader(conn)
+	writer := bufio.NewWriter(conn)
+	writeLine := func(line string) bool {
+		if _, err := writer.WriteString(line + "\r\n"); err != nil {
+			return false
+		}
+		return writer.Flush() == nil
+	}
+	if !writeLine("220 fake.test ESMTP ready") {
+		return
+	}
+	for {
+		line, err := reader.ReadString('\n')
+		if err != nil {
+			return
+		}
+		cmd := strings.TrimSpace(line)
+		srv.record(cmd)
+		upper := strings.ToUpper(cmd)
+		switch {
+		case strings.HasPrefix(upper, "EHLO"), strings.HasPrefix(upper, "HELO"):
+			ok := writeLine("250-fake.test")
+			if allowStartTLS {
+				ok = ok && writeLine("250-STARTTLS")
+			}
+			if !(ok && writeLine("250-AUTH PLAIN LOGIN") && writeLine("250 8BITMIME")) {
+				return
+			}
+		case upper == "STARTTLS" && allowStartTLS:
+			if !writeLine("220 2.0.0 ready to start TLS") {
+				return
+			}
+			tlsConn := tls.Server(conn, srv.tlsConfig)
+			if err := tlsConn.Handshake(); err != nil {
+				return
+			}
+			srv.serveUpgraded(tlsConn)
+			return
+		case strings.HasPrefix(upper, "AUTH"):
+			if !writeLine("235 2.7.0 authentication successful") {
+				return
+			}
+		case strings.HasPrefix(upper, "MAIL"), strings.HasPrefix(upper, "RCPT"):
+			if !writeLine("250 ok") {
+				return
+			}
+		case upper == "DATA":
+			if !writeLine("354 go ahead") {
+				return
+			}
+			for {
+				dataLine, err := reader.ReadString('\n')
+				if err != nil {
+					return
+				}
+				if strings.TrimRight(dataLine, "\r\n") == "." {
+					break
+				}
+			}
+			if !writeLine("250 message accepted") {
+				return
+			}
+		case upper == "QUIT":
+			_ = writeLine("221 bye")
+			return
+		default:
+			if !writeLine("250 ok") {
+				return
+			}
+		}
+	}
+}
+
+// serveUpgraded 复用命令循环处理 STARTTLS 升级后的会话（升级后不再提供 STARTTLS）。
+func (srv *fakeSMTPServer) serveUpgraded(conn net.Conn) {
+	reader := bufio.NewReader(conn)
+	writer := bufio.NewWriter(conn)
+	// net/smtp 在 StartTLS 成功后会重新发送 EHLO，直接进入命令循环即可。
+	srv.serveCommands(reader, writer)
+}
+
+func (srv *fakeSMTPServer) serveCommands(reader *bufio.Reader, writer *bufio.Writer) {
+	writeLine := func(line string) bool {
+		if _, err := writer.WriteString(line + "\r\n"); err != nil {
+			return false
+		}
+		return writer.Flush() == nil
+	}
+	for {
+		line, err := reader.ReadString('\n')
+		if err != nil {
+			return
+		}
+		cmd := strings.TrimSpace(line)
+		srv.record(cmd)
+		upper := strings.ToUpper(cmd)
+		switch {
+		case strings.HasPrefix(upper, "EHLO"), strings.HasPrefix(upper, "HELO"):
+			if !(writeLine("250-fake.test") && writeLine("250-AUTH PLAIN LOGIN") && writeLine("250 8BITMIME")) {
+				return
+			}
+		case strings.HasPrefix(upper, "AUTH"):
+			if !writeLine("235 2.7.0 authentication successful") {
+				return
+			}
+		case strings.HasPrefix(upper, "MAIL"), strings.HasPrefix(upper, "RCPT"):
+			if !writeLine("250 ok") {
+				return
+			}
+		case upper == "DATA":
+			if !writeLine("354 go ahead") {
+				return
+			}
+			for {
+				dataLine, err := reader.ReadString('\n')
+				if err != nil {
+					return
+				}
+				if strings.TrimRight(dataLine, "\r\n") == "." {
+					break
+				}
+			}
+			if !writeLine("250 message accepted") {
+				return
+			}
+		case upper == "QUIT":
+			_ = writeLine("221 bye")
+			return
+		default:
+			if !writeLine("250 ok") {
+				return
+			}
+		}
+	}
+}
+
+func smtpTestConfig(port int, useTLS bool) *SMTPConfig {
+	return &SMTPConfig{
+		Host:     "127.0.0.1",
+		Port:     port,
+		Username: "user",
+		Password: "pass",
+		From:     "noreply@example.com",
+		FromName: "Test",
+		UseTLS:   useTLS,
+	}
+}
+
+// 465 语义：UseTLS=true + 隐式 TLS 服务器，原有路径保持可用。
+func TestSMTPConnectionImplicitTLS(t *testing.T) {
+	srv, port := startFakeSMTPServer(t, true, false)
+	svc := &EmailService{}
+
+	if err := svc.TestSMTPConnectionWithConfig(smtpTestConfig(port, true)); err != nil {
+		t.Fatalf("expected implicit TLS connection to succeed, got: %v", err)
+	}
+	if !srv.sawCommand("EHLO") {
+		t.Fatal("expected server to receive EHLO")
+	}
+}
+
+// 587 语义（#1470/#1488 核心场景）：UseTLS=true + 明文问候的 STARTTLS 服务器，
+// 隐式 TLS 失败后必须自动降级为强制 STARTTLS 并成功。
+func TestSMTPConnectionStartTLSFallbackWhenTLSEnabled(t *testing.T) {
+	srv, port := startFakeSMTPServer(t, false, true)
+	svc := &EmailService{}
+
+	if err := svc.TestSMTPConnectionWithConfig(smtpTestConfig(port, true)); err != nil {
+		t.Fatalf("expected STARTTLS fallback to succeed, got: %v", err)
+	}
+	if !srv.sawCommand("STARTTLS") {
+		t.Fatal("expected server to receive STARTTLS command")
+	}
+	if got := srv.conns.Load(); got < 2 {
+		t.Fatalf("expected implicit TLS attempt before STARTTLS fallback (>=2 connections), got %d", got)
+	}
+}
+
+// UseTLS=true 但服务器不支持 STARTTLS：必须报错，且绝不能把凭据发到明文连接上。
+func TestSMTPConnectionMandatoryStartTLSRefusesPlaintext(t *testing.T) {
+	srv, port := startFakeSMTPServer(t, false, false)
+	svc := &EmailService{}
+
+	err := svc.TestSMTPConnectionWithConfig(smtpTestConfig(port, true))
+	if err == nil {
+		t.Fatal("expected error when server does not support STARTTLS")
+	}
+	if !strings.Contains(err.Error(), "STARTTLS") {
+		t.Fatalf("expected STARTTLS-related error, got: %v", err)
+	}
+	if srv.sawCommand("AUTH") {
+		t.Fatal("credentials must not be sent over plaintext when TLS is required")
+	}
+}
+
+// UseTLS=false + 服务器支持 STARTTLS：测试连接与发送路径一致，机会式升级后认证成功。
+// 这是 #1488 评论"测试连接不成功，发送测试邮件实际上能发"的回归用例。
+func TestSMTPConnectionOpportunisticStartTLSWhenTLSDisabled(t *testing.T) {
+	srv, port := startFakeSMTPServer(t, false, true)
+	svc := &EmailService{}
+
+	if err := svc.TestSMTPConnectionWithConfig(smtpTestConfig(port, false)); err != nil {
+		t.Fatalf("expected opportunistic STARTTLS test connection to succeed, got: %v", err)
+	}
+	if !srv.sawCommand("STARTTLS") {
+		t.Fatal("expected test connection to upgrade via STARTTLS like the send path")
+	}
+}
+
+// UseTLS=false + 服务器不支持 STARTTLS：保持明文直连（既有行为不回归）。
+func TestSMTPConnectionPlainWhenNoStartTLS(t *testing.T) {
+	srv, port := startFakeSMTPServer(t, false, false)
+	svc := &EmailService{}
+
+	if err := svc.TestSMTPConnectionWithConfig(smtpTestConfig(port, false)); err != nil {
+		t.Fatalf("expected plain connection to succeed, got: %v", err)
+	}
+	if srv.sawCommand("STARTTLS") {
+		t.Fatal("did not expect STARTTLS command when server does not advertise it")
+	}
+}
+
+// 发送路径全流程：UseTLS=true + STARTTLS 服务器（587 语义）完整走完 MAIL/RCPT/DATA。
+func TestSendEmailWithConfigStartTLSFallback(t *testing.T) {
+	srv, port := startFakeSMTPServer(t, false, true)
+	svc := &EmailService{}
+
+	err := svc.SendEmailWithConfig(smtpTestConfig(port, true), "rcpt@example.com", "subject", "<p>body</p>")
+	if err != nil {
+		t.Fatalf("expected send via STARTTLS fallback to succeed, got: %v", err)
+	}
+	if !srv.sawCommand("STARTTLS") {
+		t.Fatal("expected send path to upgrade via STARTTLS")
+	}
+	if !srv.sawCommand("DATA") {
+		t.Fatal("expected send path to reach DATA")
+	}
+}
+
+// 发送路径全流程：UseTLS=true + 隐式 TLS 服务器（465 语义）保持既有行为。
+func TestSendEmailWithConfigImplicitTLS(t *testing.T) {
+	srv, port := startFakeSMTPServer(t, true, false)
+	svc := &EmailService{}
+
+	err := svc.SendEmailWithConfig(smtpTestConfig(port, true), "rcpt@example.com", "subject", "<p>body</p>")
+	if err != nil {
+		t.Fatalf("expected send via implicit TLS to succeed, got: %v", err)
+	}
+	if !srv.sawCommand("DATA") {
+		t.Fatal("expected send path to reach DATA")
+	}
+}