You can verify independently if a mailbox is accessible using EWS with the following steps:
- Go to https://testconnectivity.microsoft.com.
- If using Microsoft 365, click the Microsoft 365 tab.
- Select Service Account Access (Developers) then click Next.
- Specify the target mailbox email address.
- Specify the service account user name. If using admin credentials on the connector, enter the exact same user name.
- Specify the service account password. If using admin credentials on the connector, enter the exact same password.
- Check Specify Exchange Web Services URL and enter the FQDN of the Exchange Web Service (EWS) Server. For example, if the EWS URL is https://mail.contoso.com/EWS/Exchange.asmx, then enter mail.contoso.com.
- If using on-premise Exchange Server, do not check "Use Exchange Impersonation." If you are using Microsoft 365, and using impersonation, do check the box.
- Check Ignore Trust for SSL.
- Click Perform Test.
- Once results are displayed, check the overall result, and click Expand All.
It may be necessary to first manage permissions.