Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!PT.CS.CMU.EDU!andrew.cmu.edu!cfe+
From: cfe+@andrew.cmu.edu.UUCP
Newsgroups: comp.emacs
Subject: Re: GNU Emacs vs. sendmail - battle of the titans
Message-ID: 
Date: Mon, 7-Dec-87 16:10:34 EST
Article-I.D.: andrew.UVilZ-y00VsLYwY5D5
Posted: Mon Dec  7 16:10:34 1987
Date-Received: Sat, 12-Dec-87 16:14:49 EST
Organization: Carnegie Mellon University
Lines: 18
In-Reply-To: <8712040000.AA05718@PTT.LCS.MIT.EDU>

RFC-822 does indicate a standard order of fields to reply to, but it isn't all 
three of those.  It's to use the Reply-To: field, if present; otherwise, the 
From: field.  You never reply to a Sender: address.

What I mean by ``reply to'' is that when a user asks to reply to a message 
(say, X), then the user's mail agent may assist the user by providing values 
for the fields in the header of a new message.  Typically, the To:, Subject:, 
and In-reply-to: (if not also References:) fields are derived from the fields 
of message X.  When the mail agent provides a value for the To: field, it 
should use the value that was in the Reply-to: field of message X, or the 
From: field of message X in case message X had no Reply-to: field.  (Whether 
the user agent makes use of the contents of the To: or CC: fields of message X 
is pretty much up to the user agent.)

This case is different from a system's automatically returning an error 
message to the sender of a piece of mail.  In that case, RFC821 and RFC822 
technically disagree, but what it's come down to is that you return the mail 
to the argument of SMTP's MAIL FROM: command, if you got the mail via SMTP.