Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Saleshandy - Saleshandy is a cold email outreach platform that enables marketing and sales teams to scale their email outreach operations seamlessly.
GetNotify - GetNotify is a free email tracking software solution.
Mailtrack.io - The best email tracking solution I've found. Free
Intelliverse Email Tracker - Intelliverse Email Tracker provides insights to increase the productivity of your emails.