By default, the Sendmail mail transport agent (MTA) does not accept network connections from any host other than the local computer. To configure Sendmail as a server for other clients, edit /etc/mail/sendmail.mc and change the DAEMON_OPTIONS line to also listen on network devices, or comment out this option entirely using the dnl comment delimiter. Then install the sendmail-cf package and regenerate /etc/mail/sendmail.cf by running the following commands:

su -c 'yum install sendmail-cf' 
su -c 'make -C /etc/mail'