In some releases of Red Hat Enterprise Linux 5, the sendmail Mail Transport Agent (MTA) accepted network connections from external hosts by default. In Red Hat Enterprise Linux 6, sendmail by default only accepts connections from the local system (localhost). To grant sendmail the ability to act as a server for remote hosts, perform one of the following steps:
To put either of these changes into effect, install the sendmail-cf package, then regenerate
/etc/mail/sendmail.cf. This is done by running the following commands:
su -c 'yum install sendmail-cf'
su -c 'make -C /etc/mail'