Why is Mail trying to connect to an unknown mail server (and how do I stop it)?

Hi - I'm worried that someone has hacked into my system so that it tries to connect to a fake mail server (perhaps to send spam)? I usually have the activity window open when Mail is running, and noticed that it was trying to connect to an unfamiliar mail server the other day - it was shown as an IP address whereas all the mail servers in my mail accounts have names rather than IP addresses ... so I copied the IP address down and searched for it (196.201.4.6), to find that it is run by malawi.net ... in Africa. I recently upgraded to Mac OS Lion, so initially I had figured that it was showing the mail server address differently, but I couldn't think of any reason that my email servers would be in Malawi!

By now I was getting worried: I checked all my mail accounts to be sure that something had not changed, and confirmed that none of my accounts used this mail server (or mentioned the IP address); and confirmed that the "Connecting to server "196.201.4.6"" activity message only appeared when I first reconnected to the internet (i.e. now when I manually clicked on "get mail") ... which suggests to me that it is a hidden process somewhere?

I don't have a virus checker, but downloaded ClamXav and ran a check on my user folders ... but found nothing.

Any thoughts on what this might be, or - if it is something malicious or externally controlled - how I can remove it?

Thanks - andy