mails stuck in queue

I have a CentOS server(CentOS release 6.5 (Final)) with Plesk.

There is an issue with the mail server as all the mails are getting stuck in the queue and some of them will go out after 1 or 2 hour. I have switched the MTA from qmail to postfix and reverse. Now it is qmail.

In both cases the mails stay in the queue also the new mails. My hostname is pingable and resolve.conf is updated with google nameservers, also I am able to ping to any domain from the server.

I have checked the iptables and for other firewall. No rules in iptables and no other firewall configured.

As the both MTA works same way, I think this is an issue with the sever.

Can somebody show some light to possible reasons why the server is not able to send emails?

Thanks in advance.