I'm just installing a new mail server, using Exim 4 and the sa-exim Debian package. Part of the configuration uses Teergrubing (or tarpitting), and while it sounds like a great idea in theory, I also understand that a lot of spammers these days make it pointless. They either get around this problem by using huge botnets (which have other advantages for the spammer) or they just end the SMTP transaction immediately after sending the DATA portion of the message.
But my question is really "is this of enough use to actually foil most spammers?" I know that there's still lots of stupid spammers out there who use unsophisticated methods to send their junk. I'm just wondering whether tarpitting is worth it or not.
I would say yes. Although most of, if not all, of the spam in a botnet comes from infected hosts (legit users), it is not wrong to slow the flow of spam down. From a users point of view they probably won't even notice that an e-mail takes a little longer to be delivered. And as you already mentioned this would foil stupid spammers. The last point is enough to make tarpitting worth the hassle.