Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Mailtrack.io - The best email tracking solution I've found. Free
Recurrr - Send recurring emails
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.
FollowUpThen - The simplest way to schedule an email reminder