envelopes - sender/recipient lists attached to messages
INTRODUCTION
Electronic mail messages are delivered in envelopes.
An envelope lists a sender and one or more recipients.
Usually these envelope addresses are the same as the
addresses listed in the message header:
(envelope) from djb to root
From: djb
To: root
In more complicated situations, though, the envelope
addresses may differ from the header addresses.
ENVELOPE EXAMPLES
When a message is delivered to several people at different
locations, it is first photocopied and placed into several
envelopes:
(envelope) from djb to root
From: djb Copy #1 of message
To: root, god@brl.mil
(envelope) from djb to god@brl.mil
From: djb Copy #2 of message
To: root, god@brl.mil
When a message is delivered to several people at the same
location, the sender doesn't have to photocopy it. He can
instead stuff it into one envelope with several addresses;
the recipients will make the photocopy:
(envelope) from djb to god@brl.mil, angel@brl.mil
From: djb
To: god@brl.mil, angel@brl.mil, joe, frde
Bounced mail is sent back to the envelope sender address.
The bounced mail doesn't list an envelope sender, so
bounce loops are impossible:
(envelope) from <> to djb
From: MAILER-DAEMON
To: djb
Subject: unknown user frde
The recipient of a message may make another copy and for-
ward it in a new envelope:
(envelope) from djb to joe
From: djb Original message
From: djb Forwarded message
To: joe
A mailing list works almost the same way:
(envelope) from djb to sos-list
From: djb Original message
To: sos-list
(envelope) from sos-owner to god@brl.mil
From: djb Forwarded message
To: sos-list to recipient #1
(envelope) from sos-owner to frde
From: djb Forwarded message
To: sos-list to recipient #2
Notice that the mailing list is set up to replace the
envelope sender with something new, sos-owner. So bounces
will come back to sos-owner:
(envelope) from <> to sos-owner
From: MAILER-DAEMON
To: sos-owner
Subject: unknown user frde
It's a good idea to set up an extra address, sos-owner,
like this: the original envelope sender (djb) has no way
to fix bad sos-list addresses, and of course bounces must
not be sent to sos-list itself.
HOW ENVELOPE ADDRESSES ARE STORED
Envelope sender and envelope recipient addresses are
transmitted and recorded in several ways.
When a user injects mail through qmail-inject, he can sup-
ply a Return-Path line or a -f option for the envelope
sender; by default the envelope sender is his login name.
The envelope recipient addresses can be taken from the
command line or from various header fields, depending on
the options to qmail-inject. Similar comments apply to
sendmail.
When a message is transferred from one machine to another
through SMTP, the envelope sender is given in a MAIL FROM
command, the envelope recipients are given in RCPT TO com-
mands, and the message is supplied separately by a DATA
command.
When a message is delivered by qmail to a single local
recipient, qmail-alias records the recipient in Delivered-
To and the envelope sender in Return-Path. It uses Deliv-
Path. It does not record envelope recipient addresses, on
the theory that they are redundant: you received the mail,
so you must have been one of the envelope recipients.
Note that, if the header doesn't have any recipient
addresses, sendmail will move envelope recipient addresses
back into the header. This situation occurs if all
addresses were originally listed as Bcc, since Bcc is
automatically removed. When sendmail sees this, it cre-
ates a new Apparently-To header field with the envelope
recipient addresses. This has the strange effect that
each blind-carbon-copy recipient will see a list of all
recipients on the same machine.
When a message is stored in mbox format, the envelope
sender is recorded at the top of the message as a UUCP-
style From (no colon) line. Note that this line is less
reliable than the Return-Path line added by qmail-alias or
sendmail.
SEE ALSO
qmail-header(5), qmail-alias(8), qmail-inject(8)