That is a extremely odd requirement and one that will be nearly impossible to enforce outside an organization. You tag this question as C#, but your code is VBA.
The only way I can think of to even attempt this is using VSTO to create two add-ons for Outlook. You can see in introduction to the topic
Walkthrough: Creating Your First Application-Level Add-in for Outlook[
^]
You will have to install the add-ons on both sides, the sender and receiver, and both will have to be using Outlook. If the receiver is outside your organization good luck getting that person to install an add-on just to receive emails from you.
What my first suggestion would be is to step back and look at why you are attempting this in the first place. If your only answer is because it is required then you need to look at your requirements. Because that requirement is almost impossible to implement in any correct form or fashion.