This may not pertain to your problem... but

I had a similar problem. Except... it was my account that was sending the message.

I was using a free POP mail server and I sent a message. The e-mail wasn't big nor did I have a virus on my computer.

It kept on sending the message over and over. I think it sent a copy every 27min or so.

I turned out that the problem was on their side. Something got hosed up in the server and it thought that it still had to send the message. So, it did what it was supposed to do... keep sending the message until it was sucessful. Well... the server never knew that the previous messages were sucessful and kept trying.

This happened for three messages and it took about a day to get cleared up. It must have sent 20+ copies of the same message. It was pretty hard to get ahold of someone to let them know there was a problem too...

Ah well. Just figured I'd throw that out there.

Maybe your spammer has something wrong with his server/relay?

In that case... check out http://www.mailwasher.net/ and get the free version of mailwasher. (If you are using POP mail) and blacklist the server it is coming from.

Problem solved. At least... problem work around.

And you can also get rid of some of your spam problem if you have one.