Just as sendmail can transport mail messages over a TCP/IP-based network, it can also receive mail that is sent to it over the network. To do this, it must be run in daemon mode. A daemon is a program that runs in the background independent of terminal control.
As a daemon, sendmail is run once, usually when your machine is booted. Whenever an email message is sent to your machine, the sending machine talks to the sendmail daemon that is listening on your machine.
To see how your system runs sendmail in daemon mode, run one of the following commands:
One typical example of what you will find is
/etc/rc.local:if [ -f /usr/lib/sendmail -a -f /etc/sendmail.cf ]; then /etc/rc.local: /usr/lib/sendmail -bd -q1h; echo -n ' sendmail'
The second line above shows that sendmail is run at boot time with a command line of
/usr/lib/sendmail -bd -q1h