Read up about the behaviour of virus' on some of the anti-virus software vendor's pages and it will explain a lot.
I would suggest the situation you are encountering is caused by propogation of the klez virus.
A feature of the klez virus is to on-send itself to another recipient using a "from" address of somebody found in the virus infected PC.
For example, assume I have your email address and that of "x" in my contact list. If my machine is infected then Klez will send YOU a message with X shown as being the sender. (In reality I was the sender).
Hence, the messages you have been receiving as "returned to sender" were in fact not sent by yourself, rather your email address was selected by Klez as being the sender on the PC of the real culprit.
