Here is my comprehensive check list of configurations and best practices to stop spam on MailEnable mail server over Plesk, I will keep adding any thing that i find. If you feel some thing missing here, please share in comments.
- Enable SMTP Authentication
- Limit outgoing mails per domain
- Only relay for authenticated senders
- Make sure to remove 127.0.0.1 from Allow relay for privileged IPs
- Enable URL and DNS black listings.