Does anyone know of a good guide for customizing the postfix configuration of a zimbra install?

Specifically, we're trying to limit the outgoing attachment ability for certain users. Postfix allows a configuration like this:

check_sender_access hash:/etc/no_attachment_users, body_checks_size_limit = 0

But I'm having difficulty installing it within Zimbra. Zimbra seems to overwrite the postfix conf file if I just try to insert this line into the postfix main.cf.

Thanks for your help...