How to associate email with its corresponding reply??

Hi

I'm trying retrieve the message Id of email so that I will be able to find out if the email notification that was sent recieved a reply or not? I'm using smtp email and anpop(vendor library) as of now to access the web server but I was not able to make any progress.
I was not able to access the Message-ID header attribute as it is read only??
I'm using asp.net(1.0) with c# in .NET framework 1.1.Please help me out.

Thanks
Kiran