Greylisting is a new popular feature for spam fighting, similar to whitelisting and blacklisting. Each time a mailbox receives an email from an unknown contact (IP), that mail is rejected with a “try again later” message. This means that all mail gets delayed at least until the sender tries again. This is where spam messages fail because they are not RFC compliant and will not be resent later.
The method of the gray list is to reject the first time, the emails from accounts that never sent you an email before. That is to say, if someone writes to you for the first time, the server with this antispam method activated, will reject the reception of said mail. If it is a normal mail server, with good intentions, it will consider it “normal”, and after a few minutes it will try to send this mail again. In this second attempt, the antispam system will let mail pass by, and from then on, it will let all the emails coming from that email address pass.