envelopes.5

linux下qmail的源码 本人加了一些注释

.TH envelopes 5
.SH "NAME"
envelopes \- sender/recipient lists attached to messages
.SH "INTRODUCTION"
Electronic mail messages are delivered in
.IR envelopes .

An envelope lists a
.I sender
and one or more
.IR recipients .
Usually these
envelope addresses are the same
as the addresses listed in the message header:

.EX
   (envelope) from djb to root
.br
   From: djb
.br
   To: root
.EE

In more complicated situations, though,
the envelope addresses may differ from the header addresses.
.SH "ENVELOPE EXAMPLES"
When a message is delivered to
several people at different locations,
it is first photocopied
and placed into several envelopes:

.EX
   (envelope) from djb to root
.br
   From: djb                          Copy #1 of message
.br
   To: root, [e-mail]
.EE

.EX
   (envelope) from djb to [e-mail]
.br
   From: djb                          Copy #2 of message
.br
   To: root, [e-mail]
.EE

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:

.EX
   (envelope) from djb to [e-mail], [e-mail]
.br
   From: djb
.br
   To: [e-mail], [e-mail], joe, frde
.EE

Bounced mail is sent back to the envelope sender address.
The bounced mail doesn't list an envelope sender,
so bounce loops are impossible:

.EX
   (envelope) from <> to djb
.br
   From: MAILER-DAEMON
.br
   To: djb
.br
   Subject: unknown user frde
.EE

The recipient of a message may make another copy
and forward it in a new envelope:

.EX
   (envelope) from djb to joe
.br
   From: djb                          Original message
.br
   To: joe
.EE

.EX
   (envelope) from joe to fred
.br
   From: djb                          Forwarded message
.br
   To: joe
.EE

A mailing list works almost the same way:

.EX
   (envelope) from djb to sos-list
.br
   From: djb                          Original message
.br
   To: sos-list
.EE

.EX
   (envelope) from sos-owner to [e-mail]
.br
   From: djb                          Forwarded message
.br
   To: sos-list                       to recipient #1
.EE

.EX
   (envelope) from sos-owner to frde
.br
   From: djb                          Forwarded message
.br
   To: sos-list                       to recipient #2
.EE

Notice that the mailing list is set up
to replace the envelope sender with something new,
.BR sos-owner .
So bounces will come back to
.BR sos-owner :

.EX
   (envelope) from <> to sos-owner
.br
   From: MAILER-DAEMON
.br
   To: sos-owner
.br
   Subject: unknown user frde
.EE

It's a good idea to set up an extra address,
.BR sos-owner ,
like this:
the original envelope sender (\fBdjb\fP)
has no way to fix bad
.B sos-list
addresses,
and of course bounces must not be sent to 
.B sos-list
itself.
.SH "HOW ENVELOPE ADDRESSES ARE STORED"
Envelope sender and envelope recipient addresses
are transmitted and recorded in several ways.

When a user injects mail through
.BR qmail-inject ,
he can supply a
.B Return-Path
line or a
.B \-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
.BR qmail-inject .
Similar comments apply to
.BR sendmail .

When a message is transferred from one machine to another through SMTP,
the envelope sender is given in a
.B MAIL FROM
command,
the envelope recipients are given in 
.B RCPT TO
commands,
and the message is supplied separately by a 
.B DATA
command.

When a message is delivered by
.B qmail
to a single local recipient,
.B qmail-local
records the recipient in
.B Delivered-To
and the envelope sender in
.BR Return-Path .
It uses
.B Delivered-To
to detect mail forwarding loops.

.B sendmail
normally records the envelope sender in
.BR 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,
.B sendmail
will move envelope recipient addresses back into the header.
This situation occurs if all addresses were originally listed as
.BR Bcc ,
since
.B Bcc
is automatically removed.
When
.B sendmail
sees this, it creates a new
.B 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
.B mbox
format,
the envelope sender is recorded at the top of the message
as a UUCP-style
.B From
(no colon) line.
Note that this line is less reliable than the
.B Return-Path
line added by
.B qmail-local
or
.B sendmail\fP.
.SH "SEE ALSO"
qmail-header(5),
qmail-local(8),
qmail-inject(8)