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Postfix MX Pattern Router Service

This service acts as a TCP lookup table for Postfix to dynamically route emails based on the MX records of the destination domain. It allows routing decisions to be made based on pattern matching against MX hostnames.

Operation

When Postfix needs to deliver an email, it queries this service with the destination domain. The service:

  1. Looks up the domain's MX records
  2. Compares them against the defined patterns in the configuration file
  3. If a match is found, it returns the corresponding relay server
  4. If no match is found, Postfix will use its default transport (usually direct delivery)

This can be useful to, for example, optimize email delivery for domains that use the Microsoft mail infrastructure by routing these emails through specialized third-party SMTP relays with established sender reputations.

Pattern Matching Behavior

The service uses substring matching for MX patterns, not exact matching. This means:

  • Patterns like protection.outlook.com will match MX records such as hotmail-com.olc.protection.outlook.com
  • You can use shorter, more generic patterns to match multiple similar MX records
  • The first pattern that matches any part of an MX record will be used
  • Patterns are checked in the order they appear in the configuration file

Please be aware that patterns are not matched against recipient domain but the MX records of that domain!

Installation

Requirements

  • Python 3.6 or higher

Setup

  1. Clone this repository:
$ git clone https://github.com/filidorwiese/postfix-mx-pattern-router.git /usr/local/bin/postfix-mx-pattern-router
$ cd /usr/local/bin/postfix-mx-pattern-router
  1. Install dependencies:
$ pip install -r requirements.txt

Or use package manager from your distribution.

  1. Create the configuration file to define your MX patterns:
$ nano /etc/postfix/postfix-mx-pattern-router.conf

Example configuration:

protection.outlook.com    relay:[office365-relay.example.com]:587
mx.microsoft              relay:[office365-relay.example.com]:587
icloud.com                relay:[icloud-relay.example.com]:587

Running as a Service

Create a Dedicated System User

For security reasons, it's recommended to run the service under a dedicated system user with minimal privileges:

# Create a system user and group without login capabilities
$ groupadd --system postfix-mx-pattern-router
$ useradd --system --no-create-home --shell /usr/sbin/nologin -g postfix-mx-pattern-router postfix-mx-pattern-router

Systemd Service

Create a systemd unit file to run the service as a daemon:

$ nano /etc/systemd/system/postfix-mx-pattern-router.service

Add the following content:

[Unit]
Description=Postfix MX Pattern Router Service
After=network.target

[Service]
ExecStart=/usr/local/bin/postfix-mx-pattern-router/postfix-mx-pattern-router.py -c /etc/postfix/postfix-mx-pattern-router.conf -p 10099 --cache-ttl 3600
Restart=on-failure
User=postfix-mx-pattern-router
Group=postfix-mx-pattern-router
StandardOutput=journal
StandardError=journal
SyslogIdentifier=postfix-mx-pattern-router
SyslogFacility=mail

[Install]
WantedBy=multi-user.target

Enable and start the service:

$ systemctl enable postfix-mx-pattern-router
$ systemctl start postfix-mx-pattern-router

Check the status:

$ systemctl status postfix-mx-pattern-router

Integration with Postfix

Add the following to your Postfix configuration (/etc/postfix/main.cf):

transport_maps = tcp:[127.0.0.1]:10099

Then reload Postfix.

Testing the Service

You can test the service directly from the command line using netcat (nc) to simulate Postfix queries:

$ echo "get user@outlook.com" | nc 127.0.0.1 10099
200 relay%3A%5Boffice365-relay.example.com%5D%3A587
$ echo "get user@gmail.com" | nc 127.0.0.1 10099
500 NO%20RESULT

The service responds with:

  • 200 followed by the URL-encoded relay server if a match is found
  • 500 NO%20RESULT if no match is found

You can also check the logs for more detailed information:

$ journalctl -u postfix-mx-pattern-router -f

License

This project is licensed under the BSD 3-Clause License - see the LICENSE file for details.