Understand Greylisting

This guide concerns Greylisting, a protective measure implemented on Infomaniak mail servers.

How Greylisting Works

Greylisting is a technique used by Infomaniak mail servers (among others) to combat spam. It relies on 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 adhering to message retransmission standards.

1. Initial Reception

When a mail server receives an email, it records three pieces of information:

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

2. Temporary Rejection

The server temporarily rejects the email delivery and sends a temporary error message (code 4xx). This means that the message is not permanently rejected, but the sending server must retry sending it 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) recorded previously, the message is accepted and delivered to the recipient.