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.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Recurrr - Send recurring emails
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
FollowUpThen - The simplest way to schedule an email reminder