Re: [9front] upas/fs: change header filters

[Lyndon Nerenberg <lyndon@orthanc.ca>]

This last set of changes seems reasonable, but we really do need
to pay attention to when and where this sort of header munging
takes place.

The underlying issue is that there are three distinct email services
in play:

  1) SMTP relay, and
  2) Submission (initial message injection)
  3) Final delivery

upas predates (2) being carved out as a distinct service.

In almost all cases, SMTP relay should *never* touch the headers,
other than to add a Received trace header.

The submission service (2) should *add* any missing required or
defacto-required headers (primarily Message-ID, Date, From), but
otherwise leave existing headers intact.

By the time you hand off to the local delivery agent (3), the message
is in the recipient's hands and all bets are off.

To do all of this properly requires significant restructuring of
how upas works.  Since I personally consider upas a botch from start
to finish I'm not willing to put any time into "fixing" it, although
others might.  I'd rather put the effort into replacing upas outright
with something that properly breaks up this functionality, and is
more in tune with the current set of email RFCs.

--lyndon