http://www.getsidekick.com/
https://chrome.google.com/webstore/detail/sidekick-by-hubspot/oiiaigjnkhngdbnoookogelabohpglmd
I've been using this on my work and personal GMail accounts this week (you can also use it with Outlook).
Essentially it tracks emails that are sent from your account (like the old-school read receipt), and will tell you if/when the recipient opens it, what they click, how many times they open it etc, all in real time via a pop up and dashboard.
If you've used MailChimp before, you'll know how valuable this tracking is for re-connecting with contacts, or following up on . I plan on installing it on the sales staff machine next week, hopefully giving them a tool for following up on their emails.
You don't have to track every email you send, just tick/untick the box before pressing 'send'.
It's limited to 200 notifications per month, or $10 per month for unlimited (it's currently unlimited anyway).