This guide explains how Greylisting works, a security measure applied to Infomaniak's email servers.

Introduction

• 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 in order to verify the legitimacy of the sender.
• This method leverages the fact that legitimate mail servers comply with standards by retrying the sending of a deferred message, while spam servers generally give up after the first failure.

Steps in the verification process

1. Initial Reception

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

• 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 returns a temporary error code (code 4xx). This indicates that the message is not permanently rejected, but that the sending server should try again later.

3. Retry by the sending server

Legitimate mail servers are programmed to retry sending the message after a certain period. Spam servers, on the other hand, generally do not bother to try again.

4. Validation and acceptance

Upon retrying, if the message comes from the same trio (sender IP, sender address, recipient address) that was previously recorded, the message is validated, accepted, and routed to the recipient.