Mailtrack.io - The best email tracking solution I've found. Free
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
EmailTracker.cc - Free Email Tracker for Gmail. Close More Deals with Comprehensive Email Tracking, Automated Follow-up Reminders, and Detailed Click Insights
FollowUpThen - The simplest way to schedule an email reminder
MailTracker - MailTracker is a mobile email tracking solution.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.