This parameter controls if Postfix will forward mail with sender-specified
routing (user[@%!]remote[@%!]site) from untrusted clients to destinations that
are blessed by the relay_domains
parameter.
By default, untrusted clients are not allowed to specify routing.
This closes a nasty open relay loophole where a backup MX host can
be tricked into forwarding junk mail to a primary MX host which
then spams it out to the world.