I have recently enabled tarpitting in one of my customer's mail servers.
Tarpitting is adding a small delay after each recipient (after a certain number of them). The idea is that a message with a few recipients goes fast, a message with many goes slow.
So, it should make spammers less efficient.
Some mail administrators say tarpitting doesn't work. That spammers, instead of sending a zillion mails over one connection, send a few over each of a zillion connections.
But a zillion connections are more expensive for the spammer! Or at least slower.
Well, I have proof that it does work. Sure, something like a per-ip limit on concurrent SMTP connections is a good complement, but even naïve tarpitting, all by itself, has a good effect.
Since I enabled it, peak message rate is down 60%, average is down 40%. Pretty good!
But a picture is worth 1000 words...