envelopes(5)                                                    File Formats Manual                                                   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 mes-
       sage 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 impossi-
       ble:

          (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)