I had the same problem too, a while ago. As geo said it usually is malware on the host PC. I ran a few scans and found the problem. I got sick of friends asking me if I was branching out into a new business venture. I placed an entry in my contacts with a made up email address, which will bounce back. This didn't fix the problem, but at least I knew when my friends would annoy me with abuse about email rubbish from my contact list. Google spyware terminator and there is a good free program which may help.
