🚀 go-pugleaf

From what I can tell, there are several mail headers that contain information
useful in determining the reply address for a message received (Sender:,
Reply-to:, Return-Path:, From: and From_).  Not all of these are provided
by mail agents I know and use, though 'mush' is capable of adding header
lines like Sender: and Reply-to:, and can be told in which order to check for
these fields in formulating the reply address.

Two (compound) questions, then:

1) What is the "standard" order in which these fields should be checked to 
   formulate a reply?  (In other words, what should I set reply_to_hdr to
   be?)  If there's not a standard, is their a best choice you mail gurus
   would recommend?  Is mush's default setting the best one for me to use?

2) Which of these fields should I set using mush's my_hdr command?  I can,
   for example, set the Sender: header to be my preferred send-to address,
   but is that better than setting Reply-to:?  Are either (or both) of these
   fields guaranteed to be untouched by mail agents, or is it the case that
   one (or both) may be re-written at will by mailers through who my mail
   passes?
   
What made me curious about this was an observation that some mail I receive
has dissimilar values for From:, From_ and Return-Path:, with few using
Sender: or Reply-to:.  In some cases, the values seem to depend on which
hosts the mail traversed, producing different sets of headers for two mail
messages to me from the same person.

What do all these fields mean?  To which ones should I give most weight
in deciding how to reply to a message, and how to tell other people the
best way to reach me?
                                         UUCP: att!cbosgd!djb
        David Bryant                           att!cborion!djb
        AT&T Bell Laboratories       INTERNET: djb@cbosgd.att.com
        Room 1B-256                            cborion!djb@att.com
        6200 East Broad Street          PHONE: (614) 860-4516
        Columbus, Ohio  43213             FAX: (614) 868-4302