You have an Organization 2010.Internal user named User1 sends a confidential message to another internal user named User2.User1 does not have any administrative privileges in theory. You need to ensure User1 can identify whether the message was successfully delivered？. What should you instruct User1 to do？（）
- AView the message headers in the sent item folder
- BOpen Microsoft Outlook and modify Read Receipt Settings
- COpen Microsoft Outlook and modify Delivery Receipt Settings
- DOpen the ECP and select Organize E-Mail