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From: barmar@mit-eddie.UUCP (Barry Margolin)
Newsgroups: net.mail.headers
Subject: Re: Re: Avoiding expansion of mail's metacharacters
Message-ID: <5063@mit-eddie.UUCP>
Date: Thu, 22-Aug-85 02:17:43 EDT
Article-I.D.: mit-eddi.5063
Posted: Thu Aug 22 02:17:43 1985
Date-Received: Sat, 24-Aug-85 14:44:02 EDT
References: <1391@cbosgd.UUCP> <2637@sun.uucp> <1043@sdcsvax.UUCP> <138@maynard.UUCP> <560@neuro1.UUCP>
Reply-To: barmar@mit-eddie.UUCP (Barry Margolin)
Organization: MIT, Cambridge, MA
Lines: 60

In article <560@neuro1.UUCP> sob@neuro1.UUCP (Stan Barber) writes:
>Perhaps I am wrong, but my understanding of headers (from the RFC 822
>point of view, at least), is that they form the envelope of the message
>and the "To:" line is for the benefit of the mailer as well
>as the recipient. The uux command line is generated from data contained in
>the header (on my systems, at least).

That doesn't contradict what has been suggested.  Just because the
originating system generates the delivery instructions (the "uux"
command line), but the receiver merely uses the uux parameters.  This
is, in effect, the envelope.

The Header is not equivalent to the envelope.  In your example, the Unix
mailer is generating the envelope from the header (as a secretary might
do if you gave her a letter and asked her to put it in an envelope and
mail it).

>I also think that it is a bit naive to assume that any site will
>just forward a message without "looking" at it. This implies that
>systems will look at the "To:" line before forwarding. 

In UUCP it is necessary for sites to look at and modify headers as they
pass through, because UUCP paths have to be rewritten.  This is not true
in the Arpa Internet, because host names are global.

In environments where SMTP (or CCITT's X.400 protocols) are used for
mail transfer, the receiving system does not need to look at the message
at all.  The recipient of a message is specified in SMTP with the "RCPT
TO:" command.  The following is well within the SMTP (RFC 821, I
believe) protocol:

MAIL FROM:
RCPT TO:
DATA
From: 
To: 
This is a message being delivered to someone
who is not in the header.
.

This might occur because UserB is a blind-copy recipient.

The right way to think about this is by analogy with paper mail.  The
envelope is the delivery instructions, analogous to the stuff you write
on the paper envelope (recipient's mailing address and sender's return
address).  The header corresponds to the inside address.  I'm sure most
of you have received mail at work which had your name on the envelope,
but which contained a letter that started with

	To: All Employees

In general, the envelope is information for the delivery agents, while
the header is information from the sender to the recipient.  In the real
world, the post office uses the return address when returning
undeliverable mail, while the recipient replies to the person named
in the letter.
-- 
    Barry Margolin
    ARPA: barmar@MIT-Multics
    UUCP: ..!genrad!mit-eddie!barmar