I can not disable the selinux as I am doing this in order make myself prepare for RHCE.. they want selinux to be in enforcing mode only...
Shall I make any changes in /etc/dovecot.conf file also?
Shall I try the iptables rule for IMAP server giben below:
iptables -t filter -A INPUT -p tcp --dport 143 -s ! 172.168.24.0/24 -d 172.168.24.2 (my system IP addres

-j REJECT.
172.168.24.0/24 is the iptables range for the network...
Please let me know if I can try this iptables rule also?