SMTP
535 Authentication Failed
The authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
When You See This Code
After submitting incorrect credentials via the AUTH command. The username or password does not match any account on the server.
How to Fix
Double-check your username and password. If using an app password (e.g., Gmail, Outlook), ensure it is current and has not been revoked. Also verify the correct SASL mechanism (PLAIN, LOGIN, etc.).