How to Postfix relay smtp

Published October 12, 2024 · Updated July 22, 2026 · Guides & Tutorials

Postfix is the most popular mail server on Linux, and one of its most useful roles is as a relay — a "null client" that hands all outbound mail to a trusted smart host instead of delivering it itself. This is the reliable way to make a server's email actually arrive. Here is how to configure a Postfix relay.

Why run Postfix as a relay

A server sending mail directly usually fails: no reputation, filtered ports, blocked IPs. Configuring Postfix to relay through an authenticated smart host gives every application on the box trusted, authenticated delivery through one clean IP.

Postfix relay configuration

In /etc/postfix/main.cf, set relayhost = [smtp-host]:587.
Enable SASL: smtp_sasl_auth_enable = yes and point smtp_sasl_password_maps at your credentials file.
Enable TLS: smtp_tls_security_level = encrypt.
Run postmap on the password file, then systemctl reload postfix.
Test with echo | mail -s test [email protected].

Lock it down: chmod 600 your SASL password file and restrict which local networks can inject mail (mynetworks). An unrestricted Postfix is an open relay waiting to be abused.

Related reading: Godaddy smtp relay · Smtp restrictions cpanel · free email tools

The smart host Postfix needs

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Frequently asked questions

How do I configure Postfix as a relay?

Set relayhost to your smart host on port 587, enable SASL authentication and TLS, run postmap on the password file, and reload Postfix.

What is a Postfix null client?

A Postfix setup that sends all outbound mail to a smart host and accepts no local delivery — ideal for app servers that only need to send.

What smart host should Postfix use?

A dedicated SMTP with its own IP and full authentication, so your relayed mail is trusted and delivered.