But, as Robert and I have pointed out, if the destination SMTP server flatly refuses the SMTP connection, then it's the job of the SENDING server, NOT the destination server, to decide what kind of message to return to the sender.
If the sending server is a legitimate server, delivering legitimate mail, that somehow got itself onto a blacklist, then it will probably return a failure notice to the (alleged) sender.
If the sending server is an open relay because the admin doesn't know any better, and it's relaying spam that came from somewhere else with forged return addresses, it will probably return a failure notice to the SMTP From address, which probably does not belong to the sender, but that's not my problem.
If the sending server is a zombie on a hacked Windows box belonging to some clueless broadband customer whose machine is spewing spam without his knowledge (which is quite common), sending a jillion failure notices to the machine owner would be a good idea, because people who allow their machines to spew spam, even unintentionally, deserve the consequences. And notifying him that his machine needs cleaning up would be good for him and the rest of the world. But it's highly unlikely that the SMTP return address on those spam messages actually is the real address of the owner of that machine. Sending a jillion failure notices to some innocent uninvolved individual is not a good way to convince people that the net needs cleaning up. It would be like throwing a rock through your neighbor's window to convince him crime is a problem. OK .. maybe that's stretching the analogy, since the rock-throwing would be clearly illegal, and returning failure notices to someone who didn't really send spam is technically legal, if the spam had his return address on it. But since you know that it's highly probable that the return address belongs to someone whose address was stolen, through no fault of his own, flooding him with spam rejection notices just to say "See how bad spam is? You should do something about it" is really bordering on harrassment.
[TELECOM Digest Editor's Note: So what are you saying, that I should cease my auto-ack which is intended to serve legitimate users in order to avoid possibly getting sued for harrassment by someone who gets a million auto-acks from me that some third person caused to be generated? That's bull ... anyway, I am essentially judgment-proof unless you can find some way to attach my social security disability checks. I have no other money to speak of. I do not intend to let good, legitimate, often times new users just hang waiting. Anyway, I get auto acks here all the time also, from spammers who have signed me up for every mailing list known to mankind. PAT]