Sendmail was the first well-known SMTP server.
The server has the following components:
The command line client sendmail has become a de facto interface because it's used by client to send email as an interface (mailx, …)
Almost all MTAs provide a sendmail interface making it the default implementation.
For instance, postfix postfix is a Sendmail-compatible MTA and therefore provides a sendmail command in its installation.
/sbin/service sendmail restart
ubuntu
sudo apt-get install sendmail
See How to send an email with the SendMail command line client ?