I woke up this morning to the same problem with one of my domain names
A buddy of mine had this same problem, but the really bad thing was that the sender was spamming ads for CP - he had the FBI knocking on his door a few days later & they took both his computers for a few days. He got them back, intact with nothing wrong or missing, but how hard would it have been for them to look at the source of the emails & see it was not coming from his ISP or POP account?
Just goes to show you that one asshole that you've probably never heard of can almost ruin your life