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?
The Ubuntu guide does not result in a open relay normally. So there was either a misunderstanding while you tested the server (e.g. you tested to send a email to a domain which was configured as local on the system instead of using a test like this one:
Or the server was a open relay before.
To give you a more detailed answer, post the contant of the /etc/postfix/main.cf file and the result of the relay test that i posted above.
Regarding Centos, I wont use that on a production system. Better use Ubuntu or Debian.