Greylisting is an anti- spam technique in which a mail server temporarily refuses an incoming e-mail message, and waits until the second try before accepting it. Standard mail servers will queue the message for a short time, and keep trying until the message is successfully sent. This is effective against spam, because spammers generally run non-standard mail servers which don't bother to retry later.

Greylisting is a new weapon to use against spam in this great war being waged upon it. With this new shielding method, by which you may block out huge amounts of spam, you are sure to please your email clients! In name as well as working, greylisting is related to whitelisting and blacklisting. What happen is that each time a given mailbox receives an email from an unknown contact (ip), that mail is rejected with a "try again later"-message (This happens at the SMTP layer and is transparent to the end user). This, in the short run, means that all mail gets delayed atleast until the sender tries again - but this is where spam looses out! Most spam is not sent out using RFC compliant MTAs; the spamming software will not try again later.(from http://greylisting.org/)

Process of routing email to a bulk folder if it is borderline spam, as determined by a receiving ISP.