The Sendmail program is a very widely used Mail Transport Agent (MTA).
MTAs send mail from one machine to another.
Sendmail is not a client program, which you use to read your e-mail.
Sendmail is a behind-the-scenes program which actually moves your
e-mail over networks or the Internet to where you want it to go.
If you ever need to reconfigure Sendmail, you'll also need to have the
sendmail.cf package installed. If you need documentation on Sendmail, you can
install the sendmail-doc package.