Greylisting, otherwise spelled as graylisting, is a security measure that users with email accounts may use against spam and spammers, except it's a lot less stringent than the "members only" approach of whitelists and the "permanently banned sender" approach of blacklists.An MTA (Mail Transfer Agent) that uses greylisting will momentarily block email from any sender that's not part of its database. If the message is a legitimate one, the server that sent the mail will, after a slight pause, attempt to send it again.
If a sufficient amount of time has passed by, then the email would be acknowledged and received. If the MTA decides that the correspondence is instead junk mail, it will not be recovered because a spammer will typically not bother to resend his spam because he's too busy sending unsolicited messages to thousands or even millions of emails a day and he cannot afford to "waste time" by retrying to send his spam to the same address. The beauty of having a greylist is that it's a good enough method to use in order to sieve through legitimate mail from spam without resorting to overly rigorous protocols that affects both solicited and unsolicited messages more often than not.
Greylisting is a wonderful alternative to blacklisting and whitelisting when it comes to filtering which email sender is a spammer and which isn't. What's more, this new listing method is just as effective at blocking humongous amounts of spam emails even while they're still at the mail server level as the blacklist method, except it allows a bit of leeway so that legitimate emails still have a chance to pass through if ever they're mistaken for spam. Also, the greylist doesn't need to resort to overkill statistical analysis or other error-prone heuristic techniques when implementing its content filtering and anti-spam solutions.
As a consequence, greylist-based implementations are much less strict and provide a lot more slack in order to optimize the mail sifting. The good news here is that the processor load and network traffic on your mail server is reduced dramatically by the security measure, so it can make your emailing experience much more streamlined and efficient than ever before.