I have installed Zimbra open source edition. I have tried :
If I enable smtp authentication on my smtp client and pass authentication, the smtp client can send mail to users in local or non local domain even it is not in the trust network.
If I disable smtp authentication on my smtp client and the smtp client outside the trust network, it cannot send mail to users in other domain, but can send mail to the users in local domain.
How to configure Zimbra to restrict the smtp client not able to send mail to the users in local domain if it is not enable smtp authentication and not in the trust network?