What is the purpose of email greylisting in email security?

The purpose of email greylisting in email security is to defer receipt of messages from unknown senders. This technique works by temporarily rejecting emails from unfamiliar or new senders, which prompts legitimate servers to retry sending the email after a short delay. Most email servers will attempt to resend messages after the deferral, while many spam servers will not. As a result, greylisting effectively reduces the amount of spam and unwanted emails that are delivered to the inbox, thereby enhancing overall email security.

This method leverages the behavior of email transmission, where legitimate servers are designed to retry sending emails, while many non-legitimate servers simply give up after the initial rejection. This differentiates between legitimate and spam senders, allowing only genuine emails to be received after they have been retried. The effectiveness of greylisting comes from its ability to impose a time-based challenge to suspect senders without permanently blocking any communications.