I had the same problem about 5-6 months ago. Somehow my msn messenger profile was checked-off to allow others to use my email (I DIDN'T DO IT) so I think MSN was hacked. MSN naturally denied it but the properties of the emails were consistent that they all originated from hotmail accounts.
There was also a 'hidden' message to others to use my email account. You have to very carefully read the email properties to find it. The only solution was to cancel my then current account, BUT with MSN your email account stays active for 90days. It sucked..
That being said, your problem also sounds like you could have the new MyDoom email worm virus. That's one of the features - returned emails.