From the comments in main.cf:
The relay_domains parameter restricts what destinations this system will
relay mail to. See the smtpd_recipient_restrictions restriction in the
file sample-smtpd.cf for detailed information.
By default, Postfix relays mail
- from "trusted" clients (IP address matches $mynetworks) to any destination,
- 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_alias_domains,
- destinations that match $virtual_mailbox_domains.
These destinations do not need to be listed in $relay_domains.