Advanced Postfix Recipient Validation with Per-Recipient SMTP Probing for Relay Domains

Description

A feature that extends ISPConfigs mail relay recipient handling by integrating Postfixs advanced recipient validation with per-recipient SMTP probing for relay domains.

Overview

When incoming mail is received for a relay domain, ISPConfig currently validates recipients against the mail_relay_recipient table. This enhancement should add a second validation layer based on the access policy for each recipient, allowing selective and automated SMTP probing against a configurable target mail system that has the correct valid recipient knowledge.

Why?

In our current setup, we transport email for multiple domains from our MX servers running ISPConfig to various Exchange server clusters. To reject unknown recipients during the initial SMTP dialog, Postfix on the ISPConfig system must be aware of all valid recipients.

There are several ways to achieve this:

• Populate the "Relay Recipient" table in ISPConfig with all valid recipients and keep it synchronized.
• Use LDAP, which can become complex when dealing with multiple downstream servers or clusters.
• Leverage Postfix Address Verification for recipient validation.

Goal

• No backscatter: Rejects unknown recipients at SMTP time instead of generating later DSNs.
• Dynamic validation: Real-time verification of recipient existence against the destination mail server.