Prevent outgoing spam

Don't get compromised. Seriously.

• Monitor your traffic. You'll understand what's normal and be able to recognize abnormal traffic.

• Shut down unnecessary daemons. If the server isn't supposed to send mail, don't run sendmail or postfix.

• Restrict SSH access and/or assign SSH a non-standard port (e.g. don't use the default port 22). If you need to use port 22, then augment with a service like DenyHosts to track and stop inbound SSH bot authentication attempts.

• Use or enforce strong passwords for yourself and your customers.

Oh, and this: Fighting Spam - What can I do as an: Email Administrator, Domain Owner, or User?

The easiest thing to do would be to block outgoing connections to Port 25 until a client "requests" that it be unblocked for their IP. There really shouldn't be anyone running a mail server from a CPanel hosting site in the first place (if they're generating e-mail that's "sent" by another server, then it should sent to that server on port 587 per RFC and has been that way since 1998).

I really wish more providers had your level of consideration, even if you don't block Destination Port 25 traffic. We appreciate the thought, and would appreciate the firewall even more.