I was using postfix on CentOS with VDA patches (http://vda.sourceforge.net/) for a long time. You can check link above to see what it adds and how to use it. It's pretty simple. Surely there are other ways to add quota support to the postfix, but I prefer it with VDA.
Because default CentOS/RHEL 6 repos ships old postfix version 2.6.6 and VDA website doesn't offers patches for 2.6 branch already. Of course you can always install from sources, but that might break your system and aren't really a good way. So I've decided to do it right way and build a correct RPM package with updated postfix and VDA patches applied.
Gladly I was able to find newer postfix package version 2.11.3 on GhettoForge repository (http://ghettoforge.org/index.php/Main_Page). Unfortunately VDA doesn't offers patches for 2.11 branch, newest is 2.10. :) But gladly the patch from latest 2.10.9 applied to 2.11.3 source without any issues. So I've updated the .spec file and built a binary packages for CentOS 6. GhettoForge mod also comes with builtin support for PostgreSQL, MySQL, sqlite and ldap.
I've uploaded them to packagecloud.io so anyone could use them: https://packagecloud.io/nstorm/postfix-vda-el6 Installing these packages are as easy, as running couple of commands to add a custom repo and install a package, thanks to packagecloud.io (https://packagecloud.io/nstorm/postfix-vda-el6/install):
curl -s https://packagecloud.io/install/repositories/nstorm/postfix-vda-el6/script.rpm.sh | sudo bash
Then you just install postfix and optionally perl scripts:
yum install postfix postfix-perl-scripts
Source RPM are also available.