Alain,
I think you're trying to use DSC for something that it was not designed for. Don, please correct me if I'm wrong. My understanding is that DSC has been designed to configure, monitor and correct the state of computers or devices. That is why the agent is running as local system to make local changes on computers or connect to devices related to the computer like network switches.
The configuration drift you're trying to prevent in Exchange sounds more like a rights or delegation issue. If only an configuration management system or handful of trained admins would be able to create and change mailboxes you wouldn't require another system to monitor configuration drift and change values back. If you don't have a configuration management system deployed you could still setup a scheduled task on a central system which runs a PowerShell controller script with the correct credentials to enumerate the mailboxes every day and revert changes of specify settings back to expected values.
Just my 2 cents.
Best,
Daniel