Probably want this installed,
yum install sendmail sendmail-cf m4
Now edit /etc/mail/sendmail.mc uncommenting out:
Though you’re probaby locked down and protected from external servers using your sendmail (don’t be an open relay) you can still lock down who can send by adding FEATURE(`relay_hosts_only’)dnl above the mailer options:
Now add your hosts e.g.
hostname >> /etc/mail/relay-domains
Recompile, restart, enable;
m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
systemctl restart sendmail
systemctl enable sendmail
You can also add/update your SPF record to auth your server to send mail as your domain.