Symptoms

Code ErrorAcccssDenied
Message: Access is denied. Check credentials and try again. REST APIs for this mailbox are currently in preview. You can find more information about the preview REST APIs at https://dev.outlook.com/.

Cause

The following error might appear if a hybrid environment is used. You can check more info about it via the link:
https://learn.microsoft.com/en-us/graph/hybrid-rest-support
https://techcommunity.microsoft.com/t5/exchange-team-blog/the-end-of-the-rest-api-for-on-premises-mailboxes-preview/ba-p/3221219

I.e. mixed Microsoft Azure with Active Domain on-premise. Therefore, these accounts don't exist on Azure but exist on the Exchange server on-premise. As MS Graph is working only with Azure users, it is not possible to authenticate with users on-premise AD. 

Resolution

As an alternative, we recommend setting up IMAP OAuth 2.0 in such cases or switching to O365 instead of a hybrid environment