🚀 go-pugleaf

I have an implementation similar in some respects to the one
that Kevin Crowston mentioned.  My system (a Honeywell CP-6 mainframe)
talks to the universe via CISL (aka CISL-SERVICE-MULTICS.ARPA).
We're known to CISL as "LADC" (also as HIS-LA-CP6.ARPA, but I don't
like to use that name since we are NOT connected to the ARPANET itself).

I use a hybrid approach:

-  In the "From:" field in the RFC822 header, my SMTP mailer uses the
   construct "persons-name%LADC@CISL-SERVICE-MULTICS.ARPA".  Anyone on
   the ARPANET can respond to this address (unless their mailer has a
   serious local limitation on address length) because it points to
   a registered ARPA host (CISL).

-  In the RFC 821 (SMTP) conversation with CISL, the mailer names itself
   "LADC".  For example,

         HELO LADC
         MAIL FROM:<Dave-Platt@LADC>

   This results in the eventual creation of a return-path which looks
   (to the destination host) like:

         MAIL FROM:<@CISL-SERVICE-MULTICS.ARPA:Dave-Platt@LADC>

   According to my reading of the SMTP spec, this is quite legitimate and
   results in an address that any RFC821/822-compliant mailer should be
   able to use as a return-address, should it desire to do so.  If the
   mailer tries to "short cut" the delivery (bypassing CISL and trying
   for LADC directly) it won't work, of course.  SMTP doesn't seem to
   require that the original sender identify itself (in the HELO and
   MAIL FROM commands) with a full ARPANET-usable %-hack, but only with
   a hostname that's recognized by the host it's speaking with.

The RFC822 header seen by the receiver generally looks like:

   Received: from CISL-SERVICE-MULTICS by (whomever) (date) (time)
   Received: from LADC by CISL-SERVICE-MULTICS.ARPA (date) (time)
   From: Dave Platt <Dave-Platt%LADC@CISL-SERVICE-MULTICS.ARPA>
   Return-path: <@CISL-SERVICE-MULTICS.ARPA:Dave-Platt@LADC>

Naturally, this varies with the characteristics of the receiving
mailer(s).