First, the only time I have actually used sendmail is on our very outdated work server, which comes with Sendmail as standard and do not have a package manager.
This guide has been created by configuring a Red Hat Enterprise Linux ES release 4 (Nahant).
To configure Sendmail, we need to locate sendmail.cf, the main configuration file.
In this particular install, it is located at /etc/mail/sendmail.cf
Open it in your favorite editor, which is vim and search for DS.
Edit so it looks something like this:
# "Smart" relay host (may be null) DSsmtp.company.com