How to track email open/forward events with Mac Mail (or standalone App)?
The mail protocol does not mandate that the other end acknowledge a message.

Most of these programs send rich text mail with a link to a web page embedded in them.  Loading the email into a browser fetches the link.  By keying the link to the web page to the user's email address, you can track who opened it.

So you set up your web server so that it has a virtual host with a rewrite rule that rewrites every request to a single 1 bit GIF file. But you log the request first.

Spam is a lot harder to detect.  Some mail servers will bounce it back.  More and more however just silently drop it.  Your best bet here is to try every month for 3 months, then delete it from your records.