MailTrack The best email tracking solution I’ve found. Free.
Install the Mailtrack Chrome Extension
And within seconds you will have the green ✓✓ on your Gmail once someone has read your email!
TIP: It adds “Sent by Mailtrack” in your signature so if you want to take the signature off by default, you must go to https://mailtrack.io/es/dashboard/settings/edit and unmark “Signature”.