`relay_domains`

This parameter restricts what client hostname domains (and subdomains thereof) this mail system will relay mail from, and restricts what destination domains (and subdomains thereof) this system will relay mail to.

Specify a list of hosts or domains, `/file/name` patterns or `type:name` lookup tables, separated by commas and/or whitespace. A file name is replaced by its contents; a `type:name` table is matched when a (parent) domain appears as lookup key.

NOTE: Postfix will not automatically forward mail for domains that list this system as their primary or backup MX host. See the `permit_mx_backup` restriction, in the description of the `smtpd_recipient_restrictions` parameter.