Easiest way to transfer mail setting from GPL to PRO

That depends .. which settings are you referring to exactly?

Well, there's a lot of components involved in the email system, so the answer is that it depends on the specific setting you're looking to migrate as well as what settings you have setup :-)

If there are any settings that make reference to specific files that were uploaded or created (such as an SSL certificate), that would cause a problem if you were to copy that config over without the corresponding SSL cert.

Also, this would only work if you're using the same distribution/version. It looks like you're using CentOS 5 -- this would only work if CentOS 5 was on both servers.

Lastly, I'd suggest making backups of all your config files before overwriting them.

Assuming that you don't have any custom SSL settings, or that you reconfigure them on the destination server, you can probably copy your /etc/dovecot.conf file with no trouble, that will copy over any POP/IMAP settings you have configured.

You could also copy your /etc/postfix/master.cf file, that would copy any changes you've made to SMTPS and Submission... though, it may be simpler in that case to just uncomment SMTPS and Submission on the new server, since that's generally the only change that needs to be made.

It would not work to copy your main Postfix config (/etc/postfix/main.cf) -- that would cause problems, since the hostname and such are listed in it.

If you made changes to your SpamAssassin config, those would be in "/etc/mail/spamassassin", and that can be copied.

Again though, remember that it's difficult to make generalizations like I'm doing above, it really does depend on what specifically you have setup, and what you're looking to migrate... while what I listed above should work, it might not. So be prepared with a backup of your config files so that you can revert in the case that there's a problem.