Tar pitting - a way to reduce spam

This is an interesting approach to reducing spam. The basic idea is to slow down SMTP server responses to incoming mail commands, such as that spammers either give up on the connection or are left hanging around waiting to complete the transaction for a long period. This article is a good  summary of how it works. It also suggests the idea originated in Germany, although I have seen other, apparently independent, developments along the same lines (although I can't remember where).

Basically, spammers need to send lots of mail. The more the merrier. They send millions. Often they will not wait for slow SMTP servers because they reduce their productivity. Tar pitting often results in spam just not being sent (via that server) at all because of this. Even if they do wait for slow connections the slow down reduces the throughput of the spammer. Less spam, overall, is being sent. As you can see from the article above, aggressive tar pitting can keep a spammer tied up for days waiting to complete a mail transaction. The more they are tied up trying to send, the more it costs them in time and therefore money. And that hurts spammers because their only goal is to make money, not spend it.