Can't send email in Plesk: SMTP error (454) Authentication failed

Symptoms

• Email delivery fails:

  SMTP error (454) Authentication failed

• Entries like these are logged to /var/log/maillog

  psa-pc-remote[67569]: Unable to open database(readonly) /var/lib/plesk/mail/handlers/handlers.db: unable to open database file
  psa-pc-remote[67569]: Unable to get before-data handlers list

Cause

The mail handler database has been corrupted, and can't be accessed anymore.

Resolution

Regenerate the mail handler's database to resolve the issue.

1. Connect to the server via SSH

2. Run this command to repair the affected database:

   # plesk repair mail -y