.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, god@brl.mil

.EE

.EX

   (envelope) from djb to god@brl.mil

.br

   From: djb                          Copy #2 of message

.br

   To: root, god@brl.mil

.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 god@brl.mil, angel@brl.mil

.br

   From: djb

.br

   To: god@brl.mil, angel@brl.mil, 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 god@brl.mil

.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)