So your users need access to Exchange and data from outside your organisation. Sure you can set up RPC over HTTPS and various other tools to access the data. I just find OpenVPN very good, convenient and reliable.
And after battling to find a good simple HOWTO, I put this together. It’s a quick and nasty but it works!:)
Firstly, ensure you are root, and just in case the OpenVPN is not in the base repository, add the rpmforge repo (these steps you can find elsewhere).