By default, Postfix relays mail:

<ul>
<li> from trusted clients whose IP address matches
     <a href=opt_mynetworks><tt>$mynetworks</tt></a>,
<li> from trusted clients matching <tt>$relay_domains</tt> or subdomains thereof,
<li> from untrusted clients to destinations that match <tt>$relay_domains</tt>
     or subdomains thereof, except addresses with sender-specified routing.
</ul>

The default <tt>relay_domains</tt> value is
<a href=opt_mydestination><tt>$mydestination</tt></a>.

<p>

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

<ul>
<li> destinations that match
     <a href=opt_inet_interfaces><tt>$inet_interfaces</tt></a>,
<li> destinations that match
     <a href=opt_mydestination><tt>$mydestination</tt></a>,
<li> destinations that match
     <a href=opt_virtual_maps><tt>$virtual_maps</tt></a>.
</ul>