Unfortunately Outlook simply doesn't have this capability. The feature you're looking for is typically more the realm of Contact Resource Management software. Microsoft does have the resources to design the feature you're asking for, and it pretty much exists in the Microsoft Dynamics CRM product. Dynamics CRM is able to integrate with Outlook and attach tracking codes to emails sent to contacts that are tracked within CRM. When a recipient replies to an email, it can be automatically recorded within CRM under that contact's record.

Other CRM programs such as ACT and Salesforce have similar capabilities. Dynamics CRM does have an advantage in that it is able to integrate more closely with Outlook, and can actually be accessed directly from Outlook through it's Outlook plugin.

While Outlook can be considered pretty advanced and capable for an email application, it is still at it's heart a personal email/scheduling application. The tracking you need is where more advanced contact management programs come in.

-SW