Adding a network enables you to assign policies to email originating from or going to those networks. You may use CIDR format to match large segments.
Configuration > Networks > Action > Add
Flow is the direction relative to the Mailborder server. For example, if one of your internal servers is talking to the Mailborder server, the flow is FROM. If the Mailborder server is delivering something to an external server, the flow is TO.
Priority defines in what order policies should be matched. Lower priority numbers have precedence over higher priority numbers. For example, if an email passing through the Mailborder server matches a Domain policy with a priority of 80 and a Network policy with a priority of 60, the Network policy will win. The email will then be processed using the policies defined for that Network.