Sorry, due to the nature and urgency of this matter, I have had to resort to making a new installation and trying again.
This time around I will be following the guide of yours:
Then once this is complete and tested, will install the web element of the server.
Luckily I am using a Virtual server and am able to switch off the current one, build another pretty quickly...
The client needs this server up quickly so I am going to try your guide above...
I would like to know, however, why after following the Ubuntu guide, having set it up directly as you said, that I am able to relay via my server from a random client, on a random ip address to ANY external email provider without any form of authentication on port 25 without glitch?
And anything you can think of could make the Centos guide work better?