Path: utzoo!mnetor!uunet!husc6!hao!woods
From: woods@hao.ucar.edu (Greg Woods)
Newsgroups: comp.mail.uucp
Subject: Re: domains
Message-ID: <1091@hao.ucar.edu>
Date: 14 Jan 88 20:42:47 GMT
References: <2987@hall.cray.com>
Reply-To: woods@hao.UUCP (Greg Woods)
Organization: High Altitude Obs./NCAR, Boulder CO
Lines: 46

In article <2987@hall.cray.com> dwd@hall.cray.com (Dave DeHerder_Jr.) writes:
>WELL... I got some mail with a from that looked like:
>"From: texsun!sun!hplabs!hp-sdd!sdcsvax!ncr-sd!onion.SanDiego.NCR.COM!bethh@tundra.UUCP"
>First it is confusing because all the sites are in order except that
>"tundra.UUCP" is our nearest neighbor.  Why that is at the end is a
>mystery to me.  OK.  So I figure that this is just a foible of the
>conventions and try to create a return path and come up with:
>"To: tundra!texsun!sun!hplabs!hp-sdd!sdcsvax!ncr-sd!bethh@onion.SanDiego.NCR.COM"
>Is this correct or am I all screwed up?

  You have now changed the ultimate destination of the message. The problem is,
an address like host1!user@host2 is inherently ambiguous. Does it mean
send to host1, for a user who is on host2, or send to host2, for a user
that is on host1? The official mail addressing standard (I think it's RFC822)
does not even officially recognize bangs (!) as addressing characters.
Technically, address@host should always be sent to the host on the left
of the @-sign first. How "address" is interpreted there is not specified 
(which allows things like the % kludge to work). In this case, the first
address above is perfectly valid since it means send to tundra via UUCP and
let tundra worry about interpreting the bang path. Unfortunately, the second
address says something different. It says send to onion.SanDeigo.NCR.COM
for eventual delivery to "bethh" on host "ncr-sd", which is not where
this user is. From the first path, the user "bethh" appears to be at
onion.SanDiego.NCR.COM. Therefore the second path is incorrect. If you'd left
it in pure bang form, i.e. tundra!texsun!...!ncr-ds!onion.SanDiego.NCR.COM!bethh
it MIGHT have worked. At least it would have gotten sent to tundra which is
the first hop in the path.

>
>This mail gets bounced back to me with:
>"554 texsun!sun!hplabs!hp-sdd!sdcsvax!ncr-sd!bethh@onion.SanDiego.NCR.COM... Unknown domain COM"
>

  ..which confirms what I said earlier: it was trying to send to onion directly,
which isn't what is intended.  It looks to me as though your site does not have
a domain-compatible mailer. If it doesn't even know about ".COM" that is
very likely to be the case, in which case a pure bang path is most likely
to work from your site. Not having a domain-compatible mailer at YOUR site
does not mean you can't use domains at all; it does mean that an address
of the form "user@host.domain" isn't going to work. What you will have to
do is something like bang!path!to!smarthost!host.domain!user, which looks like
a pure bang path until it gets to smarthost, which will turn it into a domain
address. If "ncr-sd" is such a host, then the above suggested path
would work in this case.

--Greg