If you’re running a mail service on a web server so it can send out e-mail from contact forms, etc then you’ll want to make sure it’s relaying outbound mail through a spam filter, or it could be used to send spam if the forms get hijacked. For qmail, you just need to create /var/qmail/control/smtproutes and enter “:yourmailserver.com”, then restart. Make sure the spam filter is configured to allow the mail server to relay through it.