NOTICE: It is possible to relay mail through a system which the anti-relay rules do not
prevent: the case of a system that doesn't use FEATURE(`route_uucp') (system A) and
relays local messages to a mail hub (e.g., via LOCAL_RELAY or LUSER_RELAY) (system B).
If system B does use FEATURE(`route_uucp'), addresses of the form

  <example.net!user@local.host>

would be relayed to <user@example.net>. System A doesn't recognize `!' as an address
separator and therefore forwards it to the mail hub which in turns relays it because it
came from a trusted local host. So if a mailserver allows UUCP (bang-format) addresses,
all systems from which it allows relaying should do the same. 
