You can set a value for the Maximum percentage of failed or deferred messages a domain may send per hour setting. Configure this option under the Mail tab at Home >> Server Configuration >> Tweak Settings.
This setting provides automated rate-limiting for accounts that generate significant quantities of failed delivery attempts. You will need to specify the percentage of outgoing mail that can fail hourly. Once the account exceeds this percentage, the account will be temporarily prevented from sending mail.
A significantly high number of failed delivery attempts is a good indicator the user may be sending spam. It could also indicate a severe misconfiguration of that user's mail forwarding settings.
