Switched auth plain to auth login in smtp login to meet server's demands

Author: MinosChatzidakis

internal/mailer/mailer.go

@@ -1,6 +1,10 @@
 package mailer
 
-import "context"
+import (
+    "context"
+	"fmt"
+    "net/smtp"
+)
 
 // Attachment is a file attached to an outbound email.
 type Attachment struct {
@@ -27,3 +31,32 @@ type Mailer interface {
 type Noop struct{}
 
 func (n *Noop) Send(_ context.Context, _ Message) error { return nil }
+
+
+type loginAuth struct {
+	username, password string
+}
+
+func LoginAuth(username, password string) smtp.Auth {
+	return &loginAuth{username, password}
+}
+
+func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+    // This sends: AUTH LOGIN
+    // The server will then prompt for the username, which your Next() method will catch.
+    return "LOGIN", nil, nil 
+}
+
+func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		switch string(fromServer) {
+		case "Username:", "VXNlcm5hbWU6":
+			return []byte(a.username), nil
+		case "Password:", "UGFzc3dvcmQ6":
+			return []byte(a.password), nil
+		default:
+			return nil, fmt.Errorf("unknown challenge: %s", string(fromServer))
+		}
+	}
+	return nil, nil
+}

internal/mailer/smtp.go

@@ -136,11 +136,10 @@ 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)
+		// Our specific server refuses PLAIN and requires LOGIN
+		auth := LoginAuth(s.username, s.password)
 		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.
-			return fmt.Errorf("mailer: SMTP authentication failed")
+			return fmt.Errorf("mailer: SMTP authentication failed: %w", err)
 		}
 	}