If you see a lot of smtp connections or your server gets constantly in spam black lists, maybe you should check if some bot spams through your server. The are 2 cases of spamming through a box: 1)using unsecure php forms that bots abuse 2)using smtp connection from an outside client this artice is about ...
here you can find various ways of sending emails with .NET 2 / .Net 1 CDONTS / Persist’s AspEmail / VB / PHP etc
Not only do I block all accesses from libwww-perl, I also log what they were looking for which turns up an amazing amount of botnet hits on a daily basis just randomly hitting websites trying to find a way inside.
You can use a group policy to protect your server from known viruses that require an executable to run. For example, Sobig.F and Blaster are two fine examples.