When you send a message with an S/MIME receipt request, you request confirmation that the message was received unaltered; you also request information about who opened the message and when it was opened.
Send a single message with an S/MIME receipt request
Send all messages with an S/MIME receipt request
From the main Microsoft Outlook window, on the Tools menu, click Options, and then click the Security tab.
Select the Add digital signature to outgoing messages check box.
Select the Request S/MIME receipt for all S/MIME signed messages check box.