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Definition of Greylist

Greylist, also known as graylisting, is a technique used in email filtering to combat spam. Unlike blacklisting, which immediately rejects emails from known spammers, greylisting temporarily defers emails from unknown or suspicious senders. When an email is greylisted, the receiving mail server responds with a temporary failure message, prompting the sending server to retry sending the email later. Legitimate mail servers typically attempt redelivery, while many spam servers do not. After the initial delay, if the email is resent, it may be accepted.

Origin of Greylist

Greylisting was first proposed by Evan Harris in 2003 as a simple yet effective method to reduce spam. Initially implemented as a feature for the popular open-source mail transfer agent, Sendmail, greylisting gained popularity due to its efficiency in blocking spam without causing inconvenience to legitimate senders. Since then, greylisting has been adopted by various mail servers and spam filtering solutions worldwide.

Practical Application of Greylist

One practical application of greylisting is in email security systems. By implementing greylisting, organizations can significantly reduce the volume of spam reaching their users' inboxes. This helps in improving overall email security by reducing the risk of phishing attacks, malware distribution, and other malicious activities carried out through email. Moreover, greylisting can also alleviate the burden on email servers by reducing the amount of unwanted incoming mail they need to process.

Benefits of Greylist

Effective Spam Reduction: Greylisting effectively reduces spam by temporarily blocking emails from unknown or suspicious sources. Since many spam servers do not retry sending emails, they are effectively filtered out by the greylisting process.

Minimal False Positives: Unlike some aggressive spam filtering techniques, greylisting minimizes false positives by only delaying emails temporarily rather than outright rejecting them. Legitimate senders usually retry sending emails, allowing them to pass through the greylisting process without issue.

Resource Efficiency: By reducing the volume of spam emails that need to be processed, greylisting helps conserve valuable resources such as bandwidth, storage, and server processing power. This can lead to improved email server performance and lower operational costs for organizations.


Legitimate emails may experience a temporary delay when they are first sent to a recipient using greylisting. However, once the sending server retries the delivery, the email typically gets through without any issues.

While greylisting is highly effective at reducing spam, it may not eliminate it entirely. Some persistent spammers may retry sending their emails, eventually bypassing the greylisting process. However, greylisting remains a valuable tool in the fight against spam.

Greylisting can cause a slight delay in email delivery, typically ranging from a few minutes to several hours, depending on how frequently the sending server retries delivery. However, this delay is usually negligible for most users and outweighed by the benefits of reduced spam.


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