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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)