How do I know if the e-mail I sent someone has been read already by him / her. If I heard it correctly, it's called a read receipt. How do I use or implement something like that?
There is no way to absolutely know for sure. Some environments support it within a given enterprise (ie, if you're emailing a co-worker). But if you're mailing someone else at random on the net, you can't.
There are some tricks to attempt to get around this. Many "advertising newsletter" types things from stores send mail only in HTML and contain embedded images that point to a unique URL on their server that is mapped to you. However, even this won't give you assurance that they read it since many mail readers are configured not to load images by default, for example.
Now, lets say you could actually detect through some trickery they opened it. Could you tell they read it? Could you tell that they clicked quickly, realized they clicked on the wrong one and clicked on another one within a second? You can't.
The best thing to do is put a phrase in it that says "the magic word is dune-buggy" and then call them and ask if they know what the magic word is :-)
there is no way for this. For the emails you are interested in, I suggest you to send them as normal mails, then add a mark to them, and review after some days if they have been answered, case notm send them again. For me it works