What is greylisting?

Here's a detailed explanation of how greylisting works:

• When an email server receives an incoming message from a sender it hasn't encountered before, it responds with a temporary rejection message, often a 4xx SMTP error code. This temporary rejection informs the sending server that the delivery is delayed and prompts it to retry later.
• Legitimate email servers, following the rules of the Simple Mail Transfer Protocol (SMTP), are designed to handle temporary failures during the email delivery process. Upon receiving a temporary rejection, a legitimate email server will make another attempt to resend the email after a specific period.
• If the email is legitimate, the sending server will retry the delivery after the temporary deferral. At this point, the recipient's email server, having noted the previous rejection, will accept the email and deliver it to the recipient's inbox.
• To streamline the process for known and trusted senders, greylisting often includes whitelisting mechanisms. Once a sender has successfully delivered an email after the initial rejection, their information (sender, recipient, and IP address) may be temporarily or permanently whitelisted. This means that future emails from this sender will be accepted immediately without undergoing the greylisting process.
• The effectiveness of greylisting lies in its exploitation of the behavior of many spam systems, which often do not make a genuine effort to resend emails after encountering a temporary rejection. Legitimate email servers, however, are configured to retry deliveries, resulting in the successful transmission of genuine emails.
• While greylisting can be effective in reducing spam, it may introduce a slight delay in the delivery of emails from unknown senders. However, this delay is usually short and is considered an acceptable trade-off for the benefits of spam reduction.