Sendmail

Take a copy of /etc/mail/sendmail.cf before proceeding.

To add the desired functionality to Sendmail, edit the /etc/mail/sendmail.mc file as the root user. When finished, use the m4 macro processor to generate a new sendmail.cf by executing the following command:

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

/sbin/service sendmail restart

Check video on youtube: https://www.youtube.com/watch?v=wbp-yn6FetY