fix(mailbox): verify SMTP STARTTLS against the mail host, not the dial IP

AgentMail compose/send failed with 'cannot validate certificate for 127.0.0.1
because it doesn't contain any IP SANs': the sender dialed the local relay at
127.0.0.1:25 and net/smtp pinned the TLS ServerName to the dial host, but the
relay's cert is for mail.<host>. Reimplement smtpSend (mirrors net/smtp.SendMail)
with an overridable IMAPConfig.SMTPServerName; default it to the mail host
(AGENTBBS_MAIL_SMTP_SERVERNAME). Now we dial the loopback for relay permission
yet verify the real hostname cert — no /etc/hosts hack. setup.sh upserts the new
var. Tested against a fake SMTP server (full MAIL/RCPT/DATA flow).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/agentbbs/main.go

@@ -1481,6 +1481,10 @@ func (a *app) mailClientFor(su store.User) (*mailbox.Client, error) {
 	cfg := mailbox.IMAPConfig{
 		IMAPAddr: env("AGENTBBS_MAIL_IMAP_ADDR", a.mailHost+":993"),
 		SMTPAddr: env("AGENTBBS_MAIL_SMTP_ADDR", "127.0.0.1:25"),
+		// Dial the loopback relay but verify STARTTLS against the mail host, whose
+		// certificate it presents (the relay's cert is never for 127.0.0.1). This
+		// avoids the /etc/hosts loopback hack the transactional sender needs.
+		SMTPServerName: env("AGENTBBS_MAIL_SMTP_SERVERNAME", a.mailHost),
 		Username:       login,
 		Password:       os.Getenv("AGENTBBS_MAIL_MASTER_PASS"),
 		// Mailu's front nginx pre-authenticates against its user DB before

internal/mailbox/imap.go

@@ -24,6 +24,11 @@ type IMAPConfig struct {
 	// SMTPUser/SMTPPass default to Username/Password when empty.
 	SMTPUser string
 	SMTPPass string
+	// SMTPServerName is the TLS server name verified during STARTTLS. Set it when
+	// the dial host differs from the certificate name — e.g. dialing the trusted
+	// local relay at 127.0.0.1:25 whose cert is mail.<host>. Empty = use the dial
+	// host (the net/smtp default).
+	SMTPServerName string
 	// Plaintext dials IMAP without TLS. Used only for a co-located backend over
 	// loopback (the Mailu gateway hitting Dovecot directly on 127.0.0.1, bypassing
 	// the front's auth proxy so master-user login works) — the password never
@@ -219,7 +224,7 @@ func (t *imapTransport) Search(_ context.Context, opts SearchOptions) ([]Message
 func (t *imapTransport) Send(_ context.Context, from string, d Draft) (SendResult, error) {
 	msg, msgID := buildRFC822(from, d)
 	// SMTPUser may be empty for a trusted local relay (no AUTH).
-	if err := smtpSend(t.cfg.SMTPAddr, t.cfg.SMTPUser, t.cfg.SMTPPass, from, recipients(d), msg); err != nil {
+	if err := smtpSend(t.cfg.SMTPAddr, t.cfg.SMTPServerName, t.cfg.SMTPUser, t.cfg.SMTPPass, from, recipients(d), msg); err != nil {
 		return SendResult{}, fmt.Errorf("smtp send: %w", err)
 	}
 	// Best-effort copy to Sent so the message shows in the member's mailbox.

internal/mailbox/smtp.go

@@ -2,6 +2,7 @@ package mailbox
 
 import (
 	"bytes"
+	"crypto/tls"
 	"fmt"
 	"mime"
 	"net"
@@ -65,17 +66,54 @@ func recipients(d Draft) []string {
 	return out
 }
 
-// smtpSend submits a built message via SMTP. With a non-empty user it does
-// STARTTLS + AUTH (e.g. smtp.profullstack.com:587); with an empty user it sends
-// unauthenticated, for a trusted local relay (e.g. the co-located Postfix).
-func smtpSend(addr, user, pass, from string, rcpts []string, msg []byte) error {
+// smtpSend submits a built message via SMTP. It does STARTTLS when the server
+// offers it, verifying the certificate against serverName (or the dial host when
+// serverName is empty) — this lets us dial a trusted local relay by IP/loopback
+// while still validating its real hostname certificate. A non-empty user adds
+// AUTH; an empty user sends unauthenticated, for a relay that trusts the source.
+//
+// This mirrors net/smtp.SendMail but with an overridable TLS ServerName, which
+// SendMail does not support (it pins ServerName to the dial host).
+func smtpSend(addr, serverName, user, pass, from string, rcpts []string, msg []byte) error {
 	host, _, err := net.SplitHostPort(addr)
 	if err != nil {
 		return fmt.Errorf("smtp addr %q: %w", addr, err)
 	}
-	var auth smtp.Auth
+	if serverName == "" {
+		serverName = host
+	}
+	c, err := smtp.Dial(addr)
+	if err != nil {
+		return fmt.Errorf("smtp dial %s: %w", addr, err)
+	}
+	defer func() { _ = c.Close() }()
+	if ok, _ := c.Extension("STARTTLS"); ok {
+		if err := c.StartTLS(&tls.Config{ServerName: serverName}); err != nil {
+			return fmt.Errorf("smtp starttls (%s): %w", serverName, err)
+		}
+	}
 	if user != "" {
-		auth = smtp.PlainAuth("", user, pass, host)
+		if err := c.Auth(smtp.PlainAuth("", user, pass, serverName)); err != nil {
+			return fmt.Errorf("smtp auth: %w", err)
 		}
-	return smtp.SendMail(addr, auth, from, rcpts, msg)
+	}
+	if err := c.Mail(from); err != nil {
+		return fmt.Errorf("smtp mail from: %w", err)
+	}
+	for _, rcpt := range rcpts {
+		if err := c.Rcpt(rcpt); err != nil {
+			return fmt.Errorf("smtp rcpt %s: %w", rcpt, err)
+		}
+	}
+	w, err := c.Data()
+	if err != nil {
+		return fmt.Errorf("smtp data: %w", err)
+	}
+	if _, err := w.Write(msg); err != nil {
+		return fmt.Errorf("smtp write: %w", err)
+	}
+	if err := w.Close(); err != nil {
+		return fmt.Errorf("smtp close: %w", err)
+	}
+	return c.Quit()
 }

internal/mailbox/smtp_test.go

@@ -0,0 +1,115 @@
+package mailbox
+
+import (
+	"bufio"
+	"net"
+	"strings"
+	"sync"
+	"testing"
+)
+
+// fakeSMTP is a minimal SMTP server (no STARTTLS advertised) that records the
+// envelope and body of one delivered message, so we can exercise smtpSend's
+// dial→MAIL→RCPT→DATA→QUIT flow without real TLS.
+type fakeSMTP struct {
+	addr        string
+	ln          net.Listener
+	mu          sync.Mutex
+	from        string
+	rcpts       []string
+	body        strings.Builder
+	gotMailFrom bool
+}
+
+func newFakeSMTP(t *testing.T) *fakeSMTP {
+	t.Helper()
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	f := &fakeSMTP{addr: ln.Addr().String(), ln: ln}
+	go f.serve()
+	t.Cleanup(func() { _ = ln.Close() })
+	return f
+}
+
+func (f *fakeSMTP) serve() {
+	conn, err := f.ln.Accept()
+	if err != nil {
+		return
+	}
+	defer conn.Close()
+	br := bufio.NewReader(conn)
+	w := func(s string) { _, _ = conn.Write([]byte(s)) }
+	w("220 mock ESMTP\r\n")
+	inData := false
+	for {
+		line, err := br.ReadString('\n')
+		if err != nil {
+			return
+		}
+		if inData {
+			if strings.TrimRight(line, "\r\n") == "." {
+				inData = false
+				w("250 ok\r\n")
+				continue
+			}
+			f.mu.Lock()
+			f.body.WriteString(line)
+			f.mu.Unlock()
+			continue
+		}
+		up := strings.ToUpper(strings.TrimSpace(line))
+		switch {
+		case strings.HasPrefix(up, "EHLO"), strings.HasPrefix(up, "HELO"):
+			w("250 mock\r\n") // single line => no extensions (no STARTTLS)
+		case strings.HasPrefix(up, "MAIL FROM"):
+			f.mu.Lock()
+			f.from = strings.TrimSpace(line[len("MAIL FROM:"):])
+			f.gotMailFrom = true
+			f.mu.Unlock()
+			w("250 ok\r\n")
+		case strings.HasPrefix(up, "RCPT TO"):
+			f.mu.Lock()
+			f.rcpts = append(f.rcpts, strings.TrimSpace(line[len("RCPT TO:"):]))
+			f.mu.Unlock()
+			w("250 ok\r\n")
+		case strings.HasPrefix(up, "DATA"):
+			inData = true
+			w("354 go ahead\r\n")
+		case strings.HasPrefix(up, "QUIT"):
+			w("221 bye\r\n")
+			return
+		default:
+			w("250 ok\r\n")
+		}
+	}
+}
+
+func TestSMTPSendFlow(t *testing.T) {
+	srv := newFakeSMTP(t)
+
+	msg := []byte("Subject: hi\r\n\r\nbody text\r\n")
+	err := smtpSend(srv.addr, "", "", "", "alice@bbs.test", []string{"bob@example.com", "carol@example.com"}, msg)
+	if err != nil {
+		t.Fatalf("smtpSend: %v", err)
+	}
+
+	srv.mu.Lock()
+	defer srv.mu.Unlock()
+	if !srv.gotMailFrom || !strings.Contains(srv.from, "alice@bbs.test") {
+		t.Fatalf("MAIL FROM wrong: %q", srv.from)
+	}
+	if len(srv.rcpts) != 2 {
+		t.Fatalf("want 2 recipients, got %v", srv.rcpts)
+	}
+	if !strings.Contains(srv.body.String(), "body text") {
+		t.Fatalf("body not delivered: %q", srv.body.String())
+	}
+}
+
+func TestSMTPSendBadAddr(t *testing.T) {
+	if err := smtpSend("not-a-host-port", "", "", "", "a@b", []string{"c@d"}, []byte("x")); err == nil {
+		t.Fatal("expected error for malformed addr")
+	}
+}

setup.sh

@@ -1129,6 +1129,9 @@ if [ "$MAIL" = "1" ]; then
   upsert_env AGENTBBS_MAIL_IMAP_ADDR "127.0.0.1:14143"
   upsert_env AGENTBBS_MAIL_IMAP_PLAINTEXT "1"
   upsert_env AGENTBBS_MAIL_SMTP_ADDR "127.0.0.1:25"
+  # Dial the loopback relay but verify its STARTTLS cert against the mail host
+  # (its cert is for ${MAIL_DOMAIN}, never 127.0.0.1) — no /etc/hosts hack needed.
+  upsert_env AGENTBBS_MAIL_SMTP_SERVERNAME "${MAIL_DOMAIN}"
 
   # Cert refresher: copy Caddy's mail cert into Mailu on renewal (like news/IRC).
   install -m 0755 "${MAILU_DIR}/refresh-certs.sh" /usr/local/bin/agentbbs-mailu-certs