Spamming engines run thousands of simultaneous threads sending mail, and are bound by their upstream bandwidth. Having a thread attempt to connect to your server and sit there waiting for it to time out slows the spam volume a negligible amount. So yes, you are releasing the pollution back into the atmosphere. Only by taking it and sequestering it do you remove it from the net.

I don’t know how much spammers charge, whether they charge by the attempted address or the delivered address. If it’s by the attempted address, then tempfailing is actually making the spam-service-supplier easy money, and costing the spamming client wasted money. Not sure this is good.

If it’s by the delivered address, then yes, it is either you or them, because the spam campaign stops when it gets to the number of delivered addresses paid for. If I were a “smart” spammer, this is what I would pay for.

Alas, once tempfailing becomes very popular, it loses its effectiveness, as the spam software will start dealing with it. There are spam arms races worth fighting, but this one’s path to failure is remarkably clear, even though it is effective right now.

And it comes at some cost — it does delay your legitimate mail somewhat. For me, that’s real. I run my own SMTP and e-mail is like IM for me. I realize a lot of the rest of the world has lost that and doesn’t care.