The provided authentication credentials are invalid. Check your username and password.
SMTP error 535 means your login credentials were rejected:
The password you entered is incorrect or has been changed.
The username format may be incorrect (try full email vs just username).
If 2FA is enabled, you need an app-specific password instead of your regular password.
Too many failed attempts may have locked the account temporarily.
Double-check your username and password. Try logging into webmail to confirm they work.
If using Gmail, Outlook, or other providers with 2FA, generate an app-specific password.
Try using your full email address as username instead of just the username part.
If you've forgotten your password, use the provider's password reset feature.
535 5.7.8 Error: authentication failed
535 Incorrect authentication data
535 5.7.8 Username and Password not accepted