@peernohell
Thank you for the detailed post and response!
Getting the MIME content of an email using MS Graph API currently isn't supported for Exchange hybrid deployments, since the ability to use these REST APIs in hybrid deployments is in preview. The only available operations are - Mail, Calendar and Contacts APIs.
As you and Lydia stated above, the only way to get the MIME content of a message via Graph API is at the "Get MIME content of a message" documentation. If you'd like to submit a feature request to our MS Graph team, you can do so using their Microsoft Graph Feature Requests forum.
You can also stay in the loop with GraphAPI changes and the MS Graph Community.
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
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