CentOS iptables and port configuration for using external SMTP server

I have a web server running CentOS and a separate mail server running Microsoft Exchange. I have the web server connecting to the mail server via SMTP on port 587 to send emails.

Sometimes the connection to the mail server refuses, but it's rare. Could an intermittent failure be related to port settings? I would have thought it's all or nothing.

For the CentOS web server, what rules should I be looking to configure in the iptables if any? Do I need to add an entry for port 587 on this server? Or do I just need to allow incoming connections on the mail server for 587?

Thanks in advance.

via Chebli Mohamed