The goal of greylisting is to reduce the amount of spam that reaches user mailboxes by taking advantage of the fact that spammers do not operate fully standards-compliant mail servers.
The premise behind greylisting is that spammers use a “fire-and-forget” method in sending spam out. If an address doesn’t work, spammers don’t care and don’t bother to retry sending it.
How greylisting takes advantage of this is that the first time mail delivery is attempted, the mail server responds with a “Temporary failure, try again later” message. Legitimate mail servers will try again later, and the second time the message is let through. Spammers won’t try again and hence the message will never get through.
Greylisting enhanced mail servers are usually set up so that once your mail passes through once, your “triplet”–that is the combination of From/To/Mail Server IP–is cached, so that next time it sees the same triplet, it lets you through the first time.
We didn’t move our greylisting system over in the migration right now but may do so in the future.