Provided by:
qmail_1.06-4_i386 
NAME
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 forward it in a
new envelope:
(envelope) from djb to joe
From: djb Original message
To: joe
(envelope) from joe to fred
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 supply 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 commands, and the message is supplied
separately by a DATA command.
When a message is delivered by qmail to a single local recipient,
qmail-local records the recipient in Delivered-To and the envelope
sender in Return-Path. It uses Delivered-To to detect mail forwarding
loops.
sendmail normally records the envelope sender in Return-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 creates 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-local or sendmail.
SEE ALSO
qmail-header(5), qmail-local(8), qmail-inject(8)
envelopes(5)