Hi,
For me, client credential flow for this scenario worked. Unfortunately, I am not able to repro your issue.
Also looking at the error you are getting, it seems the issue is not with the Graph API or roles, but with the way your mailbox is configured.
Please try a couple of things -
- Make sure your mailbox is not inactive, soft-deleted, or is hosted on-premise.
- If you are having a hybrid setup of Microsoft 365 with an on-premise exchange server, please look at this documentation once to rule out you are not missing anything.
- Check this thread to see if this is not your scenario.
- Lastly, please check if your tenant has not configured some rules around the mailbox.
If this helps, please consider accepting it as an answer and upvoting.
Thanks!