Hmmm...I'll have to look into that. I manage the mail servers for my employer I see hundreds of pieces of SPAM come in each day. Some gets blocked by our AntiVirus/AntiSpam package, some does not. That which does not but targets a bad or defunct address generates an NDR. The thing I see is the return address for the SPAM is also bogus, so the NDR generates an NDR on the target mail system, which comes back to our servers and gets routed for review by me.
Sometimes I wonder how much of the traffic on the 'Net is just this sort of NDR volleyball.
Frelling SPAMers. A pox on the lot of 'em!