There was a bug that does not make it work. Which was fixed in 7.x Build#69 and later. But you will have to re-enable it so the fixes can take effect.
If you are using the Global Sqmail add the following in your httpd.conf so sqmail can work
php_admin_value open_basedir "/path/to/sqmail:/usr/lib/php:/usr/local/lib/php:/tmp"
If you are using an dir outside these folders to save attachments you should add it to inside php_admin_value open_basedir " " too.