What is Greylisting?

Greylisting is a method that protects email users against spam. It uses a mail transfer agent (MTA) to temporarily reject any email from a sender it does not recognize. The receiving server then requests the originating server to retry delivering the email again after a delay.

A legitimate email server will recognize this request, and if sufficient time has elapsed, the server will attempt the delivery again, and this time the receiving server will allow the recipient to receive the email.

A legitimate email server will recognize this request, and if sufficient time has elapsed, the server will attempt the delivery again, and this time the receiving server will allow the recipient to receive the email.

How does greylisting effect deliverability?

Because it's one of the most common ways that Internet service providers use to reduce spam email, a greylisting has the potential to affect email deliverability. Typically, a business’s mail server will send an email to a recipient. As stated, if the recipient’s ISP does not recognize the sender, it will temporarily block the email and request the sending server to resend the email. The ISP then expects the sending server to reattempt the delivery of that email. If a business’s mail server then attempts to deliver the email from a different IP address, that server will be greylisted, so it’s vital that the retry attempt is sent from the same IP address.

Because it's one of the most common ways that Internet service providers use to reduce spam email, a greylisting has the potential to affect email deliverability. Typically, a business’s mail server will send an email to a recipient. As stated, if the recipient’s ISP does not recognize the sender, it will temporarily block the email and request the sending server to resend the email. The ISP then expects the sending server to reattempt the delivery of that email. If a business’s mail server then attempts to deliver the email from a different IP address, that server will be greylisted, so it’s vital that the retry attempt is sent from the same IP address.

Why is greylisting used?

As it appears from the above, greylisting is an effective way to prevent spam mail from reaching a recipient’s email inbox. In simple terms, it’s an easy way to check if the sender is a spam producer and, as a result, keeps recipients’ email inboxes free from spam.

Its function goes further than protecting against spam, though, and it also largely eliminates the problem that many email users face of legitimate emails ending up in their spam folders, because their spam filters wrongly classify them. This means that recipients get their mail when they’re supposed to while successfully blocking spam.  

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