Forwarding mail to an internal Exchange server

We use Zimbra as our Pop server that resides on a colocated server. We have Microsoft exchange setuo on an internal server to have the calandar features available.

I would like to forward the pop e-mail from the external Co-location to the internal Exchange server so that I can eliminate the direct usage of the pop server on internal machines. They will still use the pop server when off site.

If I can recieve mail into the exchange server and then send back thru the pop server I will be able to eliminate the confusion the girls have with personal folders/Archive Folders and Exchange Folders down to just exchange.

We are also using a combination of Office 2003 and Office 2007 on clients.
I know that putting the exchange server on the colo would solve the whole problem but due to some politics in the office I am not allowed to do that. I am stuck wioth a Zimbra Pop Solution outside and the Exchange inside...which is only being used for copmmone and shared calanaders.

Any help with the configuration would be appreciated....we use PF sense as our firewall.