Once I have a problem with a CentOS server (Could be the same on RedHat based). The HOSTNAME given on the instalation was not the reverse lookup name.
I solved with the help of the linux list, here is how. I edited the /etc/hosts
[root@ns1 ~]# cat /etc/hosts # Do not remove the following line, or various programs # that require network functionality will fail. 127.0.0.1 ns1.somedomain.com ns1 localhost.localdomain localhost
and also make sure that the info on /etc/sysconfig/network is the same.
[root@ns1 ~]# cat /etc/sysconfig/network NETWORKING=yes HOSTNAME=ns1.somedomain.com
and on the reverse lookup zone, you should have assigned
ns1.somedomain.com to the IP of the Eth in the server you have just configured.
If you do not have this way, your email server could probably not send emails from itself.
hope could be usefull.