This error usually appears when an email client tries to send a message before completing SMTP authentication correctly.
In most cases, the outgoing mail server settings are incomplete or the username and password are not being accepted by the server.
To resolve the issue, confirm that SMTP authentication is enabled in the mail client and that the outgoing server name, port, username, and password are correct.
If the issue continues, re-check the full mail server settings for the mailbox and test again.
