This is probably NOT spam, something almost as annoying though. I would check some of the relay testing sites to verify that you don't have an open relay.
If you aren't an open relay then whats happening is some spammer is sending a message to a non existant mailbox on your system using a faked/forged return address, your system attempts to send a non delivery receipt to that faked/forged address that also doesn't exist. Exchange prevents infinite mail looping by stripping the senders address and placing a <>. If it didn't strip the address your system would keep sending ndr's back and forth, because neither address is valid. Unfortunately, unless you want to turn of the NDR's completely (not an option in my organization) there really isn't an easy solution to combatting this. Eventually these buggers just time out and get deleted anyway.