diff --git a/README.md b/README.md
index 69924989..51f4bafa 100644
--- a/README.md
+++ b/README.md
@@ -107,7 +107,7 @@ type Session struct{}
 
 func (s *Session) AuthPlain(username, password string) error {
 	if username != "username" || password != "password" {
-		return errors.New("Invalid username or password")
+		return smtp.ErrAuthFailed
 	}
 	return nil
 }
diff --git a/backend.go b/backend.go
index 59cea3aa..d61f3f66 100644
--- a/backend.go
+++ b/backend.go
@@ -5,6 +5,11 @@ import (
 )
 
 var (
+	ErrAuthFailed = &SMTPError{
+		Code:         535,
+		EnhancedCode: EnhancedCode{5, 7, 8},
+		Message:      "Authentication failed",
+	}
 	ErrAuthRequired = &SMTPError{
 		Code:         502,
 		EnhancedCode: EnhancedCode{5, 7, 0},