Exact response pattern
535 5.7.8 Authentication credentials invalidPermanentGenericAuthentication
What it means
The SMTP username, password or authentication authorization failed.
Likely causes
- Wrong credentials.
- Account locked.
- Unauthorized authentication identity.
Recommended response
- Verify the exact username.
- Reset or rotate credentials.
- Check account status and allowed sending identities.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.