Have you tried looking into HTTP? That was my problem that I mentioned earlier. I had a poorly written PHP script :o for sending mail. Someone was able to inject their own email into it and used my server to send out a bunch of spam.
I would look at your HTTP logs and also do a “top” to see which process are being used that should give you some more clues.
Possibly do a netstat to see which ports are open too?
I don’t have any PHP mail scripts accesible to the public, only one in a member area of a site in beta test! No links to the beta area accesible anywhere!