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.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
FollowUpThen - The simplest way to schedule an email reminder
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Rebump - Automatic follow up emails. Keeps on bumping until you get a reply.