> I am a very new user to Majordomo. I don't know anything about mail
> transport, and like to learn about it.
The following is taken directly from my working copy of the "Majordomo
List-Owner Manual"; constructive criticisms are very welcome.
This particular chunk of text is based on a contribution from Dave Sill
(to be precise, he gave me permission to use the post on this subject
which he made on this list in May.)
-- snip --
On the internet, e-mail is transferred from one computer to another
by a method called SMTP, which means "Simple Mail Transport Protocol".
This protocol is defined and explained in a document called RFC821.
This and other e-mail related RFCs are available online from
http://www.imc.org/rfcs.html
In order to understand how internet mailing lists work, it is
important to know that SMTP is kind of like snail mail in that you
have a letter (message) inside an envelope. The letter has the
addresses of the sender and recipient(s) (e.g., the To/From/CC header
fields) but the envelope has the true sender/recipient addresses used
to deliver the letter. In the case of SMTP, the envelope addresses are
conveyed by the SMTP MAIL (sender or return path) and RCPT (recipient)
commands.
Just like the post office uses the envelope addresses to control
delivery or return undeliverable letters, so SMTP uses its envelope
addresses. If a message can't be delivered to one of the recipients,
an error message (commonly called a "bounce" message) is sent to the
sender (return path).
Let's look at some typical scenarios.
Case 1: User-to-user
User A sends a message to user B. The From header field is A, the To
header field is B, the envelope return path is A, and the envelope
recipient is B. E.g.:
(Envelope)
Sender: A
Recipient: B
(Message)
From: A
To: B
body of message
Case 2: User-to-user with BCC
User A sends a message to user B and BCC's user C. The From field is
A, the To field is B, the envelope return path is A, and the envelope
recipients are B and C:
(Envelope)
Sender: A
Recipients: B, C
(Message)
From: A
To: B
body of message
Case 3: User-to-list
User A sends a message to list LIST. List mail is a little tricky
because the message is first delivered to the mailing list resender,
then forwarded by the resender to the subscribers. The first hop is
like Case 1 except the Recipient/To is the list submission address:
(Envelope)
Sender: A
Recipients: LIST
(Message)
From: A
To: LIST
body of message
The second hop is like Case 2 except the Recipients are the
subscribers and the Sender is the list owner alias:
(Envelope)
Sender: LIST owner
Recipients: LIST subscribers
(Message)
From: A
To: LIST
body of message
Note that by setting the Sender on the second hop to the list owner
address, bounces to undeliverable recipients are directed to the list
owner rather than the originator of the message. Unfortunately, some
MTA's (Mail Transport Agents, programs which use SMTP to transport
mail from from one computer to another) aren't fully SMTP-compliant,
and send error messages to an address taken from the From: or To:
headers of the list.
In the first of these cases (error messages sent to the From: address)
the error message is often very confusing for user A, who might think
that somehow something went wrong with his post to the mailing list.
The second case (error messages sent to the To: address) is much more
serious, because the To: address is the mailing list submission
address, where people submit messages for distribution to all subscribed
addresses. Clearly it must never happen that such error messages get
redistributed to all subscribed addresses, as that would not only annoy
all subscribers once, but probably also cause the broken MTA at the
remote host to generate another error message and send it also to the
mailing list submission address. These events would repeat again
and again in a loop. Such mail loops are one of the worst things which
can happen to a mailing list; see section ??? below for a detailed
discussion of possible precautions to prevent mailing loops. Some
of these precautions are built into Majordomo while others can be
added outside of Majordomo by the maintainer.
-- snap --
May blessings from the eternal God surprise and overtake you!
Norbert.
--
Norbert Bollow, Zuerich, Switzerland. Backup e-mail address: NB@POBOX.COM
References:
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