Everything I ever care to learn about mail systems...

Configure SENDMAIL to use an external relay...

https://community.webcore.cloud/tutorials/how_to_configure_sendmail_to_use_an_smtp_relay/

yum -y install sendmail sendmail-cf mutt

vim /etc/mail/sendmail.mc

dnl define(`SMART_HOST', `smtp.your.provider')dnl

/etc/mail/make

systemctl restart sendmail

Test...

echo "Test 1 from $(hostname -f)"|mail -s "Test 1 $(hostname -f)" jeff@vorpal.net

better...

echo Test | mutt -s Test -a image.jpg -- firas@fkraiem.org

Add authinfo...

Create /etc/mail/authinfo