You can verify independently whether a mailbox is accessible using EWS by performing the following steps:
- Browse to https://www.testexchangeconnectivity.com (this is a Microsoft owned tool)
- Select Service Account Access (Developers) and click "Next"
- Specify the target mailbox email address
- Specify the service account user name (if using admin credentials on your connector, enter the exact same user name)
- Specify the service account password (if using admin credentials on your connector, enter the exact same password)
- Check "Specify Exchange Web Services URL" and specify the URL (ex: https://server/EWS/Exchange.asmx)
- Do NOT check "Use Exchange Impersonation"
- Check "Ignore Trust for SSL"
- Click "Perform Test"
- Once results are displayed, check the overall result and also click on "Expand All"
You may first need to first grant permissions.