You can verify independently whether a mailbox is accessible using EWS by performing these steps:

1. Browse to https://testconnectivity.microsoft.com This is a Microsoft-owned tool.
2. If using Office 365, click on ​the Office 365 tab.
3. Select Service Account Access (Developers) and click on Next.
4. Specify the target mailbox email address.
5. Specify the service account user name (if using admin credentials on the connector, enter the exact same user name).
6. Specify the service account password (if using admin credentials on the connector, enter the exact same password).
7. Checkmark Specify Exchange Web Services URL and specify the URL (example: https://server/EWS/Exchange.asmx).
8. If using Exchange Server, do not check Use Exchange Impersonation. If you are using Office 365, do checkmark the box, if you are using impersonation.
9. Check Ignore Trust for SSL.​
10. Click on Perform Test.
11. Once results are displayed, check the overall result, and also click on Expand All.

It may be necessary first to​ grant permissions.