Some users (Accounts) do not appear in the selection list for Mail on the Redstor web app, although they do appear for OneDrive and are backing up successfully.
UserTypedefined. According to Azure Active Directory, a user can only be
Guest. Redstor does not backup
Guestusers and there is no way for us to know if the user is a valid
The admin should update the user type for the problematic users to
Member (see Microsoft's documentation for how to do this).