example.com domain hosted on a Plesk server, emails sent from
email@example.com are rejected with the following error:
551 This mail server requires authentication before sending mail from a locally hosted domain. Please reconfigure your mail client to authenticate before sending mail.
This error means that a server or client has connected to the SMTP service to send email, using a sender address domain which exists on the mail server, but has not authenticated.
In order to fix the issue, please, follow the steps mentioned in the KB article below: