http://www.faqs.org/rfcs/rfc2822.html

Addresses in these fields (BUT PROBABLY NOT SMTP FIELDS) may instead 
be a grouip construct.
(an identifier of list:addr,addr2,...;), with possibly null addresses.
I think it is legacy, though they don't say so explicitly.

From:  List of authors
Date:  Date author committed the mail (NOT XMIT TIME!)
Sender:  Single transmitter.  Only if From: doesn't contain only the transr.

Some header fields may occur > 1 time, some cannot.  See table in RFC.

From a test telnetting to Sendmail:  Same exact results from HELO or EHLO.
(<TOKENS> like this are values passed in the corresponding SMTP command).
([TOKENS] like this are values from message content headers).

    From <MAIL>  Timestamp    (without <> brackets)
    Return-Path: <MAIL>       (with <> brackets)
    Received: from <HELO> (real.sending.host [read.ip.addr])  (<>s as HELO)
    \tby mail.server.hostname (numbers???) with SMT id ID
    \tfor <RCPT>; a time stamp (<>s as in <RCPT)
    Date: a time stamp
    Message-Id: <stamp.id@mail.server.hostname>
    From: [From:]              (with comments just as given)
    To: [To:]                  (with comments just as given)

Funny thing is, the To: line is completely ignored for delivery.  Only
the SMTP RCPT is obeyed.
NOTE:  Exim does not set the Message-ID, but (a) it does set it if mailx
sends to Sendmail without a network daemon; (b) Exim does set Return-Path:
I think that most client, including mailx, hide Return-Path: from you.


The senders (telnet and mailx) change input \n's to \r\n.  When sendmail
receives, it translates \r\n's to \n's in mailbox.  (tcpdump shows that
it goes over the network ast \r\n's.

exim adds SIZE to MAIL command, like "MAIL FROM:<b@x.n> SIZE=1385".
exim puts no space after : in SMTP commands.

MUAs (both mailx and Thunderbird mail) change recipient "add1@x.com add2@y.com"
to "RCPT TO:<add1@x.com>" and "RCPT TO:<add2@x.com>".  In Thunderbird, it
doesn't matter if multiple recipients are listed on one "To" line or 
multiple "To:" lines.

mailx actually invokes "/path/to/sendmail -i <RCPT-TO-ADDR>"
and sends the message contentx in stdin (no SMTP envelope stuff).


When putting messages into mailboxes, Sendmail adds ^> to lines beginning
with "^From ".

Mailbox message delimiters are "\nFrom " and EOF,
NOT "\n\nFrom "!! (it won't be that if msg contents don't end w/ "\n").