Centos does not open port/s after the rule/s are appended

Solution 1:

A fair amount of testing (see comments) has revealed that you don't have a firewall or transport problem. The client is able to connect through to pptpd on the server, and any remaining issues are application-related (and I see you've opened a separate question for that).

In passing, I'd echo Jiri's comments about nmap; it's not a good tool for testing end-to-end connectivity on a single port; telnet serverip 1723 would've been much better.

I'm only writing this up so you can accept an answer and prevent this question from floating around like a ghost ship, forever; and because negative answers are useful, too, and the diagnostic process shown may be of use to someone else at some future date.