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 the 2nd case,
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.
Do the following:
1) Start Internet Information Services Manager or open the Internet Information Services (IIS) snap-in.
2) Expand Server_name, where Server_name is the name of the server, right-click Default SMTP Virtual Server, and then click Properties.
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.