What is greylisting?
What exactly is greylisting?
Real mail servers automatically try to resend when a message is temporarily rejected; many spam senders do not. Greylisting takes advantage of that difference: unknown senders are briefly parked, and only those who politely try again get through.
Explanation
The result is that a first email from a new sender sometimes arrives a few minutes later than usual. After that, the sender is remembered and everything runs smoothly again. It's a quiet, effective extra layer alongside other filters. If legitimate mail keeps getting stuck, take a look at why does my email end up in spam.
Example
A new customer emails jij@jouwzaak.be for the first time. Because of greylisting, that message is held very briefly and delivered a few minutes later after all. If the same customer emails again afterwards, the mail comes in straight away.
Why greylisting works
The strength of greylisting lies in its simplicity: it exploits a behavioural difference between genuine and malicious senders, without you having to do anything. Real mail servers keep a message in the queue and automatically try again; many spam systems send each message only once and then give up. The only noticeable downside is that small delay on the very first message from a new sender. After that, the server recognises the address and mail comes in directly.
Related concepts
- Why does my email end up in spam?
- Back to the glossary
Does your mail sometimes fail to arrive? Why does my email end up in spam? helps you further.