uucp and preserving Reply-to: how?

[Mike Bilow]

Dale R. Worley wrote in a message to Mike Bilow:

 DRW> My impression is that the combination of UUCP (!) and
 DRW> Internet (@) addressing was handled much better 5 or so
 DRW> years ago, when hundreds of companies were "one UUCP hop off
 DRW> the Internet".  Now that everyone and his kid brother is
 DRW> directly connected, people aren't worrying about UUCP
 DRW> addressing any more.

Your recollection is different from mine.  As far as I recall, this stuff never
worked right then, and it doesn't work right now.  The only difference is that
it has become less important now that UUCP is nearly dead.

 DRW> But if I remember correctly, the convention was that

 DRW>    foo!bar at baz

 DRW> meant "send by SMTP to baz, who will send by UUCP to foo",
 DRW> and that 

 DRW>    foo!bar%baz

 DRW> meant "send by UUCP to foo, who will send by SMTP to baz. 
 DRW> Basically, the precedence is % > ! > @. That works for
 DRW> getting mail between the Internet and a one-hop-removed
 DRW> site.

You can construct pathological failures no matter what you concoct for parsing
rules.  The fundamental problem is that '!' is a left-to-right operator while
'%' and '@' are right-to-left operators.  UUCP regards '!' as special but '@'
as ordinary, while SMTP regards '@' as special but '!' as ordinary.  This
breaks down totally when people choose domains as UUCP intermediaries:

   ucbvax!relay.uu.net!mikebw at uucp.bilow.com

is a classic example of an address which cannot be validly parsed no matter
what you do with it, and I remember seeing things like this as long ago as the
late 1980s.  If you are an SMTP agent, you will try to send this to
uucp.bilow.com, but uucp.bilow.com is a UUCP-only machine.  Since you are an
SMTP machine, you can't send it to ucbvax, which is a UUCP node.  The proper
behavior is for the SMTP sender to transmit it to relay.uu.net, and this is how
things used to be kludged up using the DNS MX records -- but that results from
parsing uucp.bilow.com as the mail destination, not from parsing the mail
address itself to get relay.uu.net.  You could represent this consistently:

   ucbvax!relay.uu.net!uucp.bilow.com!mikebw

or

   mikebw%uucp.bilow.com at relay.uu.net

but it is impossible to tell this from the mail address alone, without knowing
at least some of the factual basis for the address format.
 
-- Mike