Sendmail should be installed by default when you install Red
Hat Linux. If it is not then you need to install the
Sendmail RPM's with the Red Hat distribution.

Configuring Sendmail
--------------------

1) Edit file "/etc/mail/sendmail.mc" - Look for the line:

    DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')

Change this line to:

    dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')

Save the file.

2)Make the sendmail configuration file:

# m4 /etc/mail/sendmail.mc > /etc/sendmail.cf

3)Restart Sendmail:

# /etc/rc.d/init.d/sendmail restart

to add to the above answer

  install sendmail-cf rpm

  after installing it

  just run service sendmail restart after any changes to 
sendmail.mc

above answer is true but one last step is missing 
#sendmail -d0 < /dev/null
#service sendmail restart
#chkconfig sendmail on