:: 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