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 dif-
       fer 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	photo-
       copy:

	  (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 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: 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	 enve-
       lope 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 sit-
       uation 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.