feat(smtp): auth login extension draft support

This adds support for the auth login extension draft.

smtp/auth_login.go

@@ -16,11 +16,31 @@ type loginAuth struct {
 }
 
 const (
-	// ServerRespUsername represents the "Username:" response by the SMTP server
+	// LoginXUsernameChallenge represents the Username Challenge response sent by the SMTP server per the AUTH LOGIN
-	ServerRespUsername = "Username:"
+	// extension.
+	//
+	// See: https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-xlogin/.
+	LoginXUsernameChallenge = "Username:"
 
-	// ServerRespPassword represents the "Password:" response by the SMTP server
+	// LoginXPasswordChallenge represents the Password Challenge response sent by the SMTP server per the AUTH LOGIN
-	ServerRespPassword = "Password:"
+	// extension.
+	//
+	//	See: https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-xlogin/.
+	LoginXPasswordChallenge = "Password:"
+
+	// LoginXDraftUsernameChallenge represents the Username Challenge response sent by the SMTP server per the IETF
+	// draft AUTH LOGIN extension. It should be noted this extension is an expired draft which was never formally
+	// published and was deprecated in favor of the AUTH PLAIN extension.
+	//
+	// See: https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00.
+	LoginXDraftUsernameChallenge = "User Name\x00"
+
+	// LoginXDraftPasswordChallenge represents the Password Challenge response sent by the SMTP server per the IETF
+	// draft AUTH LOGIN extension. It should be noted this extension is an expired draft which was never formally
+	// published and was deprecated in favor of the AUTH PLAIN extension.
+	//
+	// See: https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00.
+	LoginXDraftPasswordChallenge = "Password\x00"
 )
 
 // LoginAuth returns an Auth that implements the LOGIN authentication
@@ -56,9 +76,9 @@ func (a *loginAuth) Start(server *ServerInfo) (string, []byte, error) {
 func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
 	if more {
 		switch string(fromServer) {
-		case ServerRespUsername:
+		case LoginXUsernameChallenge, LoginXDraftUsernameChallenge:
 			return []byte(a.username), nil
-		case ServerRespPassword:
+		case LoginXPasswordChallenge, LoginXDraftPasswordChallenge:
 			return []byte(a.password), nil
 		default:
 			return nil, fmt.Errorf("unexpected server response: %s", string(fromServer))

smtp/smtp_test.go

@@ -57,10 +57,10 @@ var authTests = []authTest{
 	},
 	{
 		LoginAuth("user", "pass", "testserver"),
-		[]string{"Username:", "Password:", "Invalid:"},
+		[]string{"Username:", "Password:", "User Name\x00", "Password\x00", "Invalid:"},
 		"LOGIN",
-		[]string{"", "user", "pass", ""},
+		[]string{"", "user", "pass", "user", "pass", ""},
-		[]bool{false, false, true},
+		[]bool{false, false, false, false, true},
 	},
 	{
 		CRAMMD5Auth("user", "pass"),