envelopes - sender/recipient lists attached to messages

          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.

          When a message is delivered to several people at different
          locations, it is first photocopied and placed into several

             (envelope) from djb to root
             From: djb                          Copy #1 of message
             To: root,

             (envelope) from djb to
             From: djb                          Copy #2 of message
             To: root,

          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,
             From: djb
             To:,, 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
             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.

          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

          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.

          qmail-header(5), qmail-local(8), qmail-inject(8)

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