That isn't what mynetworks is for. It's actually a setting to define Trusted IPs that can send messages via your server (for instance, another LAN subnet at a subsidiary). For example, if you have a single server it should only consist of the following entries:
Originally Posted by ScottChapman
which would be the loopback and your LAN subnet. Any external users to your system should authenticate against the Zimbra server and they will be allowed to send mail anywhere. There are several threads in the forums discussing problem and solutions for problems you may encounter.