Understanding Greylisting

This guide concerns Greylisting, a protective measure that is implemented on Infomaniak email servers.

Operation of Greylisting

Greylisting is a technique used by Infomaniak email servers (among others) to fight spam. It is based on the principle of temporarily delaying the delivery of an electronic message to verify the legitimacy of the sender. This method exploits the fact that spam servers often send emails without complying with message retransmission standards.

1. Initial reception

When an email server receives an e-mail, it notes three elements:

• the sender's IP address
• the sender's e-mail address
• the e-mail address of the recipient

2. Temporary refusal

The server temporarily refuses the delivery of the email and sends a temporary error message (code 4xx). This means that the message is not permanently rejected, but that the sending server must try again to send it later.

3. Retest

The legitimate email servers are programmed to try to deliver the messages again after a certain period of time. Spam servers, on the other hand, often do not bother to try again.

4. Acceptance

During the retest, if the e-mail comes from the same trio (IP of the sender, address of the sender, address of the recipient) previously noted, the message is accepted and delivered to the recipient.