a little unknown tool called Graph Explorer can be used to troubleshoot Exchange online mailboxes. The tool is accessible at https://graphexplorer.cloudapp.net
Enter O365 tenant username and password. This will connect to your O365 tenant.
Once signed in will see below string in edit box:
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Drop down and select users:
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Append string with user with issues and select get to see diagnostic result
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Scroll further down in the report to ProvisioningErrors section which should be blank as shown.
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Example with errors below:
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