I usually do not recommend Gmail add-ons but this one is pretty good. I recommend that you sign up for the free trial of Sidekick:
1. Never wonder again if your emails are being viewed.
2. Know when your emails are viewed.
3. Know when your emails are being ignored.
4. Track email stats.
Check it out. Let me know what you think!
Jock Pereira | jockpereira.com | email@example.com | 978-666-4000