I would like to set up a separate post office for our employees to send only internal mail messages and reports, but not be able to send to an external user. They would need to use the post office designated for external mail for that.

We are running GW2012. We currently have one Post office, and one tree.

Is this possible? How would they switch post offices when they needed to send external email vs internal? Would I have to license more mailboxes?(essentially doubling my user mailboxes) . Is this difficult to set up? Anyone currently doing this now?

The main goal of all of this is to ensure that my employees can send confidential info internally without "accidentally" being able to include an external address, thus compromising data security/privacy. We have a solution for sending confidential info externally though our service provider, but have gotten requests to see if there is a way to use separate mailboxes?

Any suggestions?
Thanks- Tom McBride