envelopes(5)

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)