Why don’t they hit reply?

I was looking through my sent mail yesterday, wondering if I had really sent an email to someone.. as I had yet to see a response. It turns out I had sent it several weeks ago. So, what happened at their end? Did my email get ignored? Vanish? Filtered out as spam?

Now I am left wondering how I would go about tracking my own emails to see if the person at the other end is even getting them.

Comments? Suggestions?