greylisting

Dear legit sender,
this mailserver uses greylisting as a means to fight spam. This might delay the mail you send to our domains by a few minutes (or hours) on first contact. You don't need to do anything. Your mailserver will just re-send. (If it's a good mailserver, it will re-send from the same IP address.)
Dear spambot,
talking to the greylisting and greyscanning smtp daemon we run here gives you a pretty good chance of getting trapped and ending up on several blacklists. All you need to do is try to deliver mail to an address we both know in advance does not even exist in any of our domains. Should you get tired of inventing the garbage yourself, feel free to reuse any of the spam trap addresses that other spammers have used before. Come on, do it. Prove to us that you are nothing but a spamming machine so we can stutter the whole SMTP dialogue only to reject the message anyway and eventually blacklist you. Pleased to be wasting your time, sir.
Dear mail server admin,
this server is an MX for a few domains. We run a greylisting SMTP daemon (specifically, OpenBSD's spamd) in front of the real SMTP daemon. The GREY connections are fed to greyscanner and we let it blacklist (almost) everything that differs significantly from the few valid addresses in these domains, known in advance. Feel free to use the list of currently trapped hosts as a supplement to your bigger blacklists. It is generated from the spamd database every night.
the number of white and trapped hosts