The Free On-line Dictionary of Computing (30 December 2018):

sendmail

    The BSD Unix Message Transfer Agent supporting
   mail transport via TCP/IP using SMTP.  Sendmail is
   normally invoked in the background via a Mail User Agent
   such as the mail command.

   Sendmail was written by Eric Allman at the University of
   California at Berkeley during the late 1970s.  He now has his
   own company, Sendmail Inc.

   Sendmail was one of the first programs to route messages
   between networks and today is still the dominant e-mail
   transfer software.  It thrived despite the awkward ARPAnet
   transition between NCP to TCP protocols in the early 1980s
   and the adoption of the new SMTP Simple Mail Transport
   Protocol, all of which made the business of mail routing a
   complex challenge of backward and forward compatibility for
   several years.  There are now over one million copies of
   Sendmail installed, representing over 75% of all Internet mail
   servers.

   Simultaneously with the announcement of the company in
   November 1997, Sendmail 8.9 was launched, featuring new tools
   designed to limit junk e-mail.  SendMail 8.9 is still
   distributed as source code with the rights to modify and
   distribute.

   The command

   	sendmail -bv ADDRESS

   can be used to learn what the local mail system thinks of
   ADDRESS.  You can also talk to the Sendmail daemon on a
   remote host FOO with the command

   	telnet FOO 25

   (1998-08-25)