.ref-BSD-4_3_Net_2/usr/src/usr.sbin/sendmail/doc/usenix.abs

           SENDMAIL -- An Internetwork Mail Router
                        Eric Allman
                     Britton-Lee, Inc.

Routing mail through a heterogenous internet presents many new
problems.  Among the worst of these is that of address mapping.
Historically, this has been handled on an ad hoc basis.  However,
this approach has become unmanageable as internets grow.

Sendmail acts a unified "post office" to which all mail can be
submitted.  Address interpretation is controlled by a production
system, which can parse both old and new format addresses.  The
new format is "domain-based", a flexible technique that can
handle many common situations.  Sendmail is not intended to perform
user interface functions.

Sendmail will replace delivermail in the Berkeley 4.2 distribution.
Several major hosts are now or will soon be running sendmail.
This change will affect any users that route mail through a sendmail
gateway.  The changes that will be user visible will be discussed.