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: