fix(mailer): support SMTP AUTH LOGIN when PLAIN is unavailable

CHANGELOG.md

@@ -92,6 +92,12 @@ exact tag (`ghcr.io/calnode/calnode:0.1.0`) if you need stability between upgrad
    CSP and would break the embedding this exists for.
 
 ### Fixed
+- **SMTP email works with servers that offer AUTH LOGIN but not AUTH PLAIN.**
+  Calnode previously used PLAIN for every authenticated SMTP connection, which
+  failed against servers such as Microsoft 365 that advertise `LOGIN XOAUTH2`
+  after STARTTLS. It now prefers PLAIN when offered and uses LOGIN otherwise.
+  LOGIN requires TLS, and servers offering neither supported method return a
+  clear error.
 - **Rescheduling on the manage page works again.** Its slot list called an `esc()`
   helper that only ever existed on the booking page, so any day with availability
   threw before rendering and could not be rescheduled. The helper is now defined

docs/ARCHITECTURE.md

@@ -645,6 +645,12 @@ as the desired state:
   genuinely broken SMTP config. Explicit credentials state intent. `GET /v1/settings/email`
   returns the live `transport`, and the admin UI badges it, so filled-in SMTP fields are
   never mistaken for SMTP delivery.
+- **SMTP authentication:** after the configured TLS handshake, if any, `smtp.go`
+  reads the server's advertised AUTH mechanisms. It uses PLAIN when
+  available, otherwise LOGIN if offered (as with some Microsoft 365 servers).
+  LOGIN refuses to send credentials without TLS; if neither mechanism is offered,
+  sending fails with an unsupported-authentication error. Authentication failures
+  omit the server's full response from logs to avoid exposing credential details.
 - **TCP relay:** `EMAIL_SMTP_CONNECT_HOST` / `EMAIL_SMTP_CONNECT_PORT` are loaded by `internal/config` and passed through `SMTPConfig` / `BuildMailer` at boot and settings reload. They change only the dial address; TLS and authentication keep the original SMTP host. Active overrides are logged.
 - **The dial must stay bounded.** `defaultSMTPTimeout` once applied only via
   `conn.SetDeadline`, which runs *after* the dial returns, leaving the dial itself bounded

internal/mailer/smtp.go

@@ -90,6 +90,52 @@ func newDialers(deadline time.Time, host string) (net.Dialer, tls.Dialer) {
 		}
 }
 
+// loginAuth implements the two-challenge AUTH LOGIN exchange. Credentials are
+// only sent after the connection has been upgraded to TLS.
+type loginAuth struct {
+	username string
+	password string
+	step     int
+}
+
+func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+	if !server.TLS {
+		return "", nil, errors.New("mailer: AUTH LOGIN requires TLS")
+	}
+	return "LOGIN", nil, nil
+}
+
+func (a *loginAuth) Next(_ []byte, more bool) ([]byte, error) {
+	if !more {
+		return nil, nil
+	}
+	switch a.step {
+	case 0:
+		a.step++
+		return []byte(a.username), nil
+	case 1:
+		a.step++
+		return []byte(a.password), nil
+	default:
+		return nil, errors.New("mailer: unexpected AUTH LOGIN challenge")
+	}
+}
+
+func (s *SMTP) authForMethods(methods string) (smtp.Auth, error) {
+	available := strings.Fields(strings.ToUpper(methods))
+	for _, method := range available {
+		if method == "PLAIN" {
+			return smtp.PlainAuth("", s.username, s.password, s.host), nil
+		}
+	}
+	for _, method := range available {
+		if method == "LOGIN" {
+			return &loginAuth{username: s.username, password: s.password}, nil
+		}
+	}
+	return nil, errors.New("mailer: SMTP server does not offer AUTH PLAIN or LOGIN")
+}
+
 func (s *SMTP) Send(ctx context.Context, msg Message) error {
 	addr := net.JoinHostPort(s.connectHost, s.connectPort)
 	// Built before anything is dialed, so an unusable recipient costs no connection.
@@ -150,7 +196,11 @@ func (s *SMTP) Send(ctx context.Context, msg Message) error {
 	defer c.Close()
 
 	if s.username != "" {
-		auth := smtp.PlainAuth("", s.username, s.password, s.host)
+		_, methods := c.Extension("AUTH")
+		auth, err := s.authForMethods(methods)
+		if err != nil {
+			return err
+		}
 		if err := c.Auth(auth); err != nil {
 			// Don't wrap err — SMTP auth responses can contain server-side
 			// detail that may expose credential information in logs.

internal/mailer/smtp_test.go

@@ -4,11 +4,64 @@ import (
 	"context"
 	"errors"
 	"net"
+	"net/smtp"
 	"sync/atomic"
 	"testing"
 	"time"
 )
 
+func TestSMTPAuthForMethods(t *testing.T) {
+	s := &SMTP{host: "smtp.office365.com", username: "user@example.com", password: "secret"}
+
+	// Office 365 advertises LOGIN and XOAUTH2, but not PLAIN.
+	auth, err := s.authForMethods("LOGIN XOAUTH2")
+	if err != nil {
+		t.Fatalf("select LOGIN: %v", err)
+	}
+	if _, ok := auth.(*loginAuth); !ok {
+		t.Fatalf("AUTH LOGIN XOAUTH2 selected %T; want AUTH LOGIN", auth)
+	}
+
+	// Prefer the existing mechanism when a server supports both.
+	auth, err = s.authForMethods("LOGIN PLAIN")
+	if err != nil {
+		t.Fatalf("select PLAIN: %v", err)
+	}
+	if auth == nil {
+		t.Fatal("AUTH LOGIN PLAIN selected no authentication mechanism; want PLAIN")
+	}
+	mechanism, _, err := auth.Start(&smtp.ServerInfo{Name: s.host, TLS: true})
+	if err != nil || mechanism != "PLAIN" {
+		t.Fatalf("AUTH LOGIN PLAIN selected %q (error: %v); want PLAIN", mechanism, err)
+	}
+	if _, err := s.authForMethods("XOAUTH2"); err == nil {
+		t.Fatal("XOAUTH2-only server should reject unsupported password authentication")
+	}
+}
+
+func TestLoginAuthRequiresTLSAndAnswersChallenges(t *testing.T) {
+	auth := &loginAuth{username: "user@example.com", password: "secret"}
+	if _, _, err := auth.Start(&smtp.ServerInfo{TLS: false}); err == nil {
+		t.Fatal("AUTH LOGIN accepted an unencrypted connection")
+	}
+	mechanism, initial, err := auth.Start(&smtp.ServerInfo{TLS: true})
+	if err != nil || mechanism != "LOGIN" || initial != nil {
+		t.Fatalf("Start = %q, %q, %v; want LOGIN and no initial response", mechanism, initial, err)
+	}
+	for _, want := range []string{"user@example.com", "secret"} {
+		response, err := auth.Next(nil, true)
+		if err != nil || string(response) != want {
+			t.Fatalf("challenge response = %q, %v; want %q", response, err, want)
+		}
+	}
+	if _, err := auth.Next(nil, true); err == nil {
+		t.Fatal("AUTH LOGIN accepted a third challenge")
+	}
+	if response, err := auth.Next(nil, false); err != nil || response != nil {
+		t.Fatalf("completed exchange = %q, %v; want no response", response, err)
+	}
+}
+
 // A server that accepts the TCP connection but never writes the initial SMTP greeting
 // reproduces the exact class of hang this test guards against: a port/TLS-mode mismatch
 // (e.g. STARTTLS spoken to an implicit-TLS port) or an unresponsive server, where the