What exactly is greylisting and how does it work?

Greylisting is supported by several mail server applications including Exim. This method of taking email greatly reduces the amount of incoming spam.

How does greylisting work?

1. When someone sends a message to the mail server, the mail server will record the sender, the recipient and the server IP address (also called "triplet") in a so-called greylist.
2. The receiving mail server returns a notification to the sending mail server that the message should be offered again.
3. When the message is offered again after 2 minutes up to a maximum of 8 hours and the "triplet" is the same, it is added to the whitelist. If there is no further traffic with this "triplet" it is removed from the whitelist after 30 days. When there is traffic the "triplet" remains on the whitelist.
4. For subsequent messages, the whitelist is used to pass the message immediately.

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