This guide concerns Greylisting, a protection measure implemented on Infomaniak's mail servers.

How Greylisting Works

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

1. Initial Reception

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

• the sender's IP address
• the sender's email address
• the recipient's email address

2. Temporary Rejection

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

3. Retry

Legitimate mail servers are programmed to retry delivering messages after a certain delay. Spam servers, on the other hand, often do not bother to retry.

4. Acceptance

Upon retry, if the email comes from the same trio (sender's IP, sender's address, recipient's address) noted previously, the message is accepted and delivered to the recipient.