Content-type: text/html Manpage of envelopes

envelopes

Section: File Formats (5)
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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.milangel@brl.mil

   From: djb

   To: god@brl.milangel@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)


 

Index

NAME
INTRODUCTION
ENVELOPE EXAMPLES
HOW ENVELOPE ADDRESSES ARE STORED
SEE ALSO

This document was created by man2html, using the manual pages.
Time: 02:14:38 GMT, October 23, 2009