:: What’s Postfix
Postfix is free and open source mail transfer agent.

:: How to setup Postfix relay email to Google Apps / Gmail in CentOS 6.3
+ Setup main.cf to relay email

add some configure like these:

+ Setup Google Apps / Gmail Login

add line like this:

+ Create Postfix db file for gmail login

+ set CAfile

+ Restarting Postix

+ Testing send an email

:: Links
+ Google
+ Carlosgomez