Does it look up the local_part in a $domain.aliases file, and returns a
"no" if local_part is not found, or a "yes" if it is?
I need a condition that matches both $local_part and $domain for virtual
domains, but can't think of a good way to do it. That is, if a message is
addressed to user@???, then use this router; if the address is
user@???, skip and go to next router.
