🚀 go-pugleaf

Although one would generally like to have !-paths left intact, there
is at least one circumstance wherein reroutes are entirely
appropriate.  When a site is set up to do replies using the Path:
header line due to not having INTERNET #defined, then reroutes may be
a necessary fact of life.

The reason is that not all host1!host2 connections that show up in the
Path: line are UUCP connections.  We speak NNTP with, e.g.,
mailrus.cc.umich.edu.  (Yes, Ohio State and Michigan can be civil
toward one another. :-)  So when we trade articles with them, the
Path: headers gain pseudo-UUCP connections that say
	tut.cis.ohio-state.edu!mailrus!some!where!username
and as we forward such an article farther down the line (via UUCP, for
example, through osu-cis and then to other Columbus sites), more sites
get tacked on the front of that.

Now someone replies to the !-path.  It says
	mumble!fubar!tut.cis.ohio-state.edu!mailrus!some!where!username
which hasn't got a chance of working.  We don't speak UUCP with
mailrus; we talk with them over our common NSF tributary.  So this
mail will do fine until it gets to tut.cis.ohio-state.edu, where we
will try to send mail via UUCP to mailrus - which will fail because
there is no `mailrus' in Tut's Systems file.  We don't reroute; we
just fail the mail and return it to where it came from, if possible.

This reflects not so much on the problems of !-path re-routing as it
does on the use of such an ambiguous syntax for the Path: header.  A
!-path `path' is not necessarily indicative of the transport used.