I really hate bounce-back spam! (I call it bounce-back spam, but the official name for it is Backscatter.) I've read, and been told by sysadmins, that there is not much that can be done about it. The Wikipedia page on bounce messages has a little section that explains why:
Excluding MDAs, all MTAs forward mails to another MTA. This next MTA is free to reject the mail with an SMTP error message like user unknown, over quota, etc. At this point the sending MTA has to inform the originator, or as RFC 5321 puts it:
If an SMTP server has accepted the task of relaying the mail and later finds that the destination is incorrect or that the mail cannot be delivered for some other reason, then it MUST construct an "undeliverable mail" notification message and send it to the originator of the undeliverable mail (as indicated by the reverse-path).
This rule is essential for SMTP: as the name says, it's a simple protocol, it cannot reliably work if mail silently vanishes in black holes, so bounces are required to spot and fix problems.
Today, however, most email is spam, which usually utilizes forged Return-Paths. It is then often impossible for the MTA to inform the originator, and sending a bounce to the forged Return-Path would hit an innocent third party. This inherent flaw in today's SMTP (without the deprecated source routes) is addressed by various proposals, most directly by BATV and SPF.
It looks like I'll have to just deal with it. (I could set up filters and such, but then I might miss a real bounce-back and not know that my message didn't go through!) I'm just grateful it comes in waves of a few hours every few weeks instead of non-stop! Has anyone else had to deal with this? If so, what did you do about it?