Greylisting is quickly emerging as the new weapon that can be effectively used to fight against spam.
Greylisting is somewhat related to whitelisting and blacklisting, but instead of specifying who is on your white or black lists, Greylisting works in a different way.
Each time a mailbox receives an e-mail from an unknown contact, that e-mail is rejected with a “try again later” message.
This means that the e-mail message is delayed for a few more moments until the sender tries to send it again.
You see, spammers rely on speed, and their spam-shooting applications are built to quickly bombard a domain’s MX records, but they rarely listen to the recipient’s mail server’s reply. Because of that, the spammers will not obey your server’s request for them to “try again later”.