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.

Bananatag - Find out what happens to your emails after you press send! Add email tracking, attachment tracking, email scheduling and templates to your daily emails.

FollowUpThen - The simplest way to schedule an email reminder

GetNotify - GetNotify is a free email tracking software solution.

Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.

Mailtrack.io - The best email tracking solution I've found. Free