What is Greylisting and how does it work?
Greylisting is a popular tool in the fight against spam. It will temporarily block incoming mail from a sender and then returns the mail to the sender's mail server with a message saying effectively, "try again later." The sending server must then retry sending the mail.
Greylisting is effective because spammers will not usually bother to attempt a second delivery, but legitimate e-mail servers will.
Why use Greylisting?
Greylisting is a very effective method of spam blocking that comes at a minimal price in terms of performance. Most of the actual processing that needs to be done for Greylisting takes place on the sender's server. It has been shown to block upwards of 95% of incoming spam simply because so many spammers don't use a standard mail server. As such, spam servers generally only attempt a single delivery of a spam message and don't reply to the "try again later" request.
Disadvantages of Greylisting
The biggest disadvantage of Greylisting is the delay of legitimate e-mail from servers not yet verified. This is especially apparent when a server attempts to verify a new user's identity by sending them a confirmation email.