SMTP authentication failed: how to read the failure
An SMTP authentication failure happens before normal message delivery. The mail client or application reached a submission service but could not prove it was allowed to send using the configured identity.
Confirm the exact SMTP server and port
A provider can use different hostnames for incoming mail, outgoing submission, or webmail. Use the documented submission hostname, TLS mode, and port rather than copying an IMAP setting into the SMTP field.
Verify the username format and credential
Many services require the full email address as the SMTP username. Password resets, account migrations, or credential rotation can leave a website or phone using an old value even while another client still works.
Check multi-factor and app-password requirements
Some services do not permit a normal interactive password for legacy mail protocols when MFA is enabled. Follow the provider’s current authentication method rather than weakening MFA to make an old client work.
Read the server response before retrying repeatedly
The SMTP response can distinguish bad credentials, disabled authentication, account suspension, TLS requirements, or policy blocks. Repeated blind retries can trigger additional security controls.