I am complete newbie to all of this so please forgive me if my question is basic.

I have had a website moved to linode and will be taking over the basic maintenance for it. The site is PHP/MySQL and it needs to send out emails based on customer selections. The function was not working. I couldn't find a doclib on basic email gateway support on CentOS so in the end I just started the POSTFIX service to see what would happen!

To my surprise the email out functions of the website started to work. Subsequently I read the 'Host Email with Postfix, Dovecot and MySQL on CentOS 5' doclib which contains the following statement:

The version of Postfix included in the main CentOS repository does not include support for MySQL.

I didn't follow this instruction but the website now appears to working OK sending out emails.

My first question is: Does the doclib statement only apply to version 5.0 and not 6.0 as I have installed?

Secondly, at some stage I think I may have to set-up a full email server, so can I follow all the actions in the version 5.0 doclib to get it to work on version 6.0?

I'm guessing all the SQL stuff is done in the PHP code, not by postfix itself. Postfix only needs to have support for database stuff if the recepients of e-mails are to be defined in a database and not as users on the actual system. You only send outgoing e-mails. So that reference is of no importance to you. I'd guess.