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mail                         Overview                        mail




Electronic mail system


The  COHERENT system  includes a  full-featured,  UNIX-style mail
system.  It  consists of a  number of commands  and files through
which you  can describe potential  recipients of mail,  either on
your  system  or other  systems,  and send  mail  to them  either
directly or  via UUCP.  This article describes  the design of the
COHERENT mail system,  and introduces the commands and files that
compose it.

***** Structure of the COHERENT Mail System *****

The  COHERENT mail  system has three  major components:  the _u_s_e_r
_a_g_e_n_t  (i.e., mmaaiill);  the _r_o_u_t_i_n_g _a_g_e_n_t  (the commands  ssmmaaiill and
rrmmaaiill); and the delivery agents (the commands llmmaaiill and uuuuxx).

The user agent collects messages from the user and reads messages
from  a  user's  mailbox.  It  hands  to  the  routing agent  for
delivery any messages it receives from the user.

The routing  agents decode addresses and decide  how to deliver a
message.  They  are the only  components of the  mail system that
must run sseettuuiidd to assume the privilege of the superuser rroooott.

The delivery agents move messages to their destination.

The  local delivery  agent,  llmmaaiill, places  messages into  users'
mailboxes.   To  discourage  mail  forging,  llmmaaiill does  not  use
sseettuuiidd. It  must be run by a privileged  user (generally rroooott) to
write into all mail boxes.  As a rule, llmmaaiill is invoked only by a
routing agent.

The UUCP delivery agent, uuuuxx, queues messages for transmission to
remote systems.   It uses sseettuuiidd  to assume the  identity of user
uuuuccpp in order to  write into the necessary spool directories.  It
has long  been trivial to  forge messages to  remote systems with
uuuuxx; keep this in mind if you plan to use electronic mail for any
kind of authorization system.

ssmmaaiill  looks up  each _u_s_e_r in  file /uussrr/lliibb/mmaaiill/aalliiaasseess.  If it
finds a match, it uses the matched name in place of _u_s_e_r. If _u_s_e_r
is of the form


        _s_y_s!_u_s_e_r


or







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mail                         Overview                        mail



        _s_y_s! ... !_u_s_e_r


or


        _u_s_e_r@_s_y_s[._d_o_m_a_i_n]


it  is  treated  as a  remote  destination.   ssmmaaiill then  invokes
command uuuuxx  to pass the message to  _s_y_s, whose responsibility it
becomes to pass the message to _u_s_e_r.

If   ssmmaaiill   finds   no   match  in   /uussrr/lliibb/mmaaiill/aalliiaasseess,   or
$HHOOMMEE/.aalliiaasseess, however, it attempts to look up each _u_s_e_r in file
/eettcc/ppaasssswwdd, to see if this is a local user.  If it does not find
_u_s_e_r  in this  file, ssmmaaiill  mails  an error  message back  to the
sender.   If, however,  it  does find  _u_s_e_r in  this file,  ssmmaaiill
checks   file   $HHOOMMEE/.ffoorrwwaarrdd   for  any   possible   forwarding
instructions.  If  this file is absent,  ssmmaaiill passes the message
to   llmmaaiill.   llmmaaiill    writes   the   message   into   the   file
/uussrr/ssppooooll/mmaaiill/_u_s_e_r.    This   file   is   called   the   user's
``mailbox''.  Note  that _u_s_e_r owns  this file, and  can therefore
permit or deny access to her mail by other users.

***** Files *****

$HHOOMMEE/ddeeaadd.lleetttteerr -- Message that mmaaiill could not send
$HHOOMMEE/.aalliiaasseess -- Personal mail alias file for outgoing mail
$HHOOMMEE/.ffoorrwwaarrdd -- Forwarding instructions for inbound mail
/eettcc/ppaasssswwdd -- User identities
/ttmmpp/mmaaiill* -- Temporary and lock files
/uussrr/lliibb/mmaaiill/aalliiaasseess -- Aliases of users
/uussrr/lliibb/mmaaiill/ffuullllnnaammeess -- Short full name aliases of users
/uussrr/lliibb/mmaaiill/ppaatthhss -- Mail routing control file
/uussrr/ssppooooll/mmaaiill -- Mailbox directory, filed by user name

***** See Also *****

aalliiaasseess, ccoommmmaannddss, mmaaiill, mmkkffnnaammeess, mmssgg, nnppttxx, ppaatthhss, uuuuxx

***** Notes *****

To  mail a  file  to another  user, use  the shell's  redirection
operator `<'.  For example, the command


        mail stephen <bug.report


mails file bbuugg.rreeppoorrtt to  user sstteepphheenn. The file will be prefixed
with  your address,  and suffixed  with your  mail ``signature'',
should you have one.




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