According to the documentation, there are only two situations where you can access other users' mail. However, the scope of shared mailboxes is limited to people within the organization and cannot be shared for accounts in other domains, so if you need to access mail from other domain users, you need to use application permissions.
More details can be found at this link:
https://learn.microsoft.com/en-us/graph/api/user-list-messages?view=graph-rest-1.0&tabs=http
Hope this helps.
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