What you can do: if you have any links inside the mail, add a token to them that you can count by a script, like http://.......mycontent.pl?userid=49152
These are people who actively click on a link, i.e. they are potentially interested in your content.
People who aren't, can't and shouldn't be counted, like dorward just pointed out. You won't get the accurate number, and you're going to bother them.

As a sender of bulk mail, you should be extremely careful about what you do, otherwise you will find your company in spam filtering lists quickly. Email marketing already has a very bad reputation these days, so please be careful not to make things worse!