You can have a deep understanding of your users and how they use mails by this:
- each mail gets an ID stored in database (mailid,...., openrate)
- you input this line in your mail templates "<img src="open.php?id=mailid>" which 1. returns an empty clear image 2. updates openrate+1 for this mailid in database
- Whenever a user opens the mail and the mail is displayed with image-display-on, the openrate for this mailid increases by 1
- You can see which users open which templates and how often.