Email spoofing

E-mail spoofing is a term used to describe fraudulent e-mail activity in which the sender address and other parts of the e-mail header are altered to appear as though the e-mail originated from a different source (from Wikipedia).

If you have received undelivered emails warnings regarding email you never sent, it is possible you are the victim of a spammer who is using your email for his mass-mail distribution. The messages you are getting are those that could not reach destination, unfortunately they are being sent back to you rather than the spammer because of the spoofing.

Unfortunately there is nothing that you can do but delete these warning emails and wait it out.