flexfax: faxmail generates wasted DNS lookups

[David Woolley <david@djwhome.demon.co.uk>]

In answering another question, I realised that there is a flaw in the
instructions for installing faxmail under sendmail.

They provide a special rule for ruleset 0, but what they don't do is
provide a rule in ruleset 3 to bypass canonicalisation.  Without such
a rule, the default sendmail configuration will do a DNS lookup for
every mailed fax.  What's more, that lookup will fail and therefore only
be cached for the very short time used for negative caching.  Even worse,
any forwarders will not cache it, and as it is a fake top level domain,
it will eventually reach a root nameserver or an unofficial secondary
for one - this is rather anti-social behaviour.

Specifically, with the distributed configurations for sendmail, fax
needs to be in class P.