Well I agree to Sergey that doing something such like that is not easy and you cannot be sure whether it is send or not. I would rather not to go that way you asked but if you really do need to:
Here is an approach that you might want to try:
You can create an httphandler for your website images. If you send an HTML message which includes at least 1 image, then you could embed querystring data to the end of that image. This could even be something like a 1x1 transparent image. When the user reads the email, this sends the request to the server to fetch the image data, and in turn, you could capture that request and denote that the message was read.
This is not bulletproof however, because most email clients block images by default unless the user specifies they would like to view images in the email.
A link that you might want to read:
So you'd like to send some email through code