The idea is that spammers don't use real mail queues and thus, if you send a temporary error, genuine clients will requeue the message, whereas spammers will ju

The idea is that spammers don't use real mail queues and thus, if you send a temporary error, genuine clients will requeue the message, whereas spammers will just drop it. Over time, this assertion has become false and now it's just annoying. Do you have some evidence for this claim? Greylisting heavily reduced the amount of spam I received when I started using greylisting years ago. I still see large amounts of spam rejected, so I'd like to hear more about how it doesn't work any more - it seems to reduce the spam-level significantly for me.