Are you getting hundreds of UNDELIVERABLE MESSAGE reports?
Here's how to resolve the problem.
1) Immediately scan the computer for viruses and/or trojans. In addition to anti-virus software, use a program such as Spybot Search and Destroy
2) Change the password to something very secure.
3) If it wasn't the mailbox that was compromised, it could just be that a spammer randomly chose your email to receive the bounces. In this case, just delete them as they come in and it should pass in a day or two.
Let us know if you need further assistance to resolve.