Protect existing Data with Azure RMS

Hi

I configured successfully Azure RMS in my organisation. Now I got a new requirement from the security team to protect all existing data in my Exchange DBs with RMS. For Files and Sharepoint I can use the bulk tool, but for Exchange I cant find a solution. For my understanding every emails need to get through the transport service to protect it and its not possible to do it direct on the store level. Is there any solution to protect existing files with RMS in my Exchange organisation?

Thanks for your help!

Regards
Joerg