usr/src/usr.lib/sendmail/doc/abstract

Here it is, on the fly.....

        SENDMAIL -- An Internetwork Mail Router

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 domain-based addressing and old-style
ad hoc addresses.  Mail is then dispatched to an outgoing mailer.
This system can expand trivially.  The production system is powerful
enough to rewrite addresses in the message header to conform to the
standards of a number of common target networks, including old
(NCP/RFC733) Arpanet, new (TCP/RFC822) Arpanet, UUCP, and Phonenet.
Sendmail is not intended to perform user interface functions or
final delivery.  Sendmail also implements an SMTP server, message
queueing, and aliasing.

This is approach is unique in that it allows external compatibility
with the old practices, and tries to make the mail system conform to
the user instead of the other way around.  Although sendmail is not
intended to circumvent new standards, it is intended to make the
transition less painful.  Sendmail does require certain base-level
standards on target mailers such as the basic semantics of certain
headers and the surface syntax of messages.  New mailers can be added
trivially; for example, a Purduenet channel was brought up in twenty
minutes.