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>

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")
+	}
+}