Ahmad Imanudin has a nice series of tutorials on installing and configuring policyd on Zimbra.
- Install and enable web ui.
2. Password protect web ui with .htaccess
3. Configure policyd using web ui
Zimbra Tips : How To Configure Rate Limit Sending Message on PolicyD
4. Reject unlisted domain using policyd web ui
https://imanudin.net/2014/09/11/improving-anti-spam-reject-unlisted-domain-on-zimbra-8-5/