At one time, the only Mail Transfer Agent (MTA) software easily available
was sendmail(8).  As a result of this, most Mail User Agents (MUAs) such
as mail(1) had the path and calling conventions expected by sendmail(8)
compiled in.

Times have changed, however.  On a modern NetBSD system, the administra-
tor may wish to use one of several available MTAs.

It would be difficult to modify all MUA software typically available on a
system, so most of the authors of alternative MTAs have written their
front end message submission programs so that they use the same calling
conventions as sendmail(8) and may be put into place instead of
sendmail(8) in /usr/sbin/sendmail.
