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Greylisting is a method of email spam protection and exists to protect users of email services from spam.
A server that uses greylisting stores the following data of each incoming email message:
- the IP address of the sending mail server
- the sender email address
- the recipient email address
This collection of information is known as a "triplet".
Every triplet is compared against an internal database of the receiving mail server.
If a triplet has not occurred within a configured time frame, the relevant email message is "greylisted" for a configured time frame - usually 300 secods = 5 minutes.
This means that the receiving mail server does not accept the message for 5 minutes.
A correctly configured mail server will attempt to delivery the message again after the time frame (5 minutes). Then the message is accepted by the receiving mail server and can be delivered to the recipient.
Most mail servers that are used for spam distribution are not configured according to RFC and so a server that sent a message that was not accepted initially will not attempt to deliver the message again. As such, greylisting enables very effective spam protection! |
