This executes Mail Server Daemon which runs as a daemon in the background, listening for incoming mail from other machines.
Sendmail can handle both incoming and outgoing mail. Sendmail routes mail for the user to the proper delivery program based on the email address. It accepts mail from a user’s mail program, interprets the mail address, rewrites the address into the proper form for the delivery program, and routes the mail to the correct delivery program. Sendmail insulates the end user from these details. If the mail is properly addressed, sendmail will see that it is properly passed on for delivery. Likewise, for incoming mail, send mail interprets the address and either delivers the mail to a user’s mail program or forwards it to another system.
/etc/init.d/sendmail {start|stop|restart|condrestart|status}
/etc/mail/sendmail.cf