fixes #19634 (#20545)

api/libs/smtp.py

             else:
                 smtp = smtplib.SMTP(self.server, self.port, timeout=10)
 
-            if self.username and self.password:
+            # Only authenticate if both username and password are non-empty
+            if self.username and self.password and self.username.strip() and self.password.strip():
                 smtp.login(self.username, self.password)
 
             msg = MIMEMultipart()