By default, Postfix relays mail:

- from trusted clients whose IP address matches `$mynetworks`,
- from trusted clients matching `$relay_domains` or subdomains thereof,
- from untrusted clients to destinations that match `$relay_domains` or subdomains thereof, except addresses with sender-specified routing.

The default `relay_domains` value is `$mydestination`.

In addition to the above, the Postfix SMTP server by default accepts mail that Postfix is final destination for:

- destinations that match `$inet_interfaces`,
- destinations that match `$mydestination`,
- destinations that match `$virtual_maps`.