FollowUpThen - The simplest way to schedule an email reminder

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.

Simply Remind Me - Free email and text reminder service so you don't forget the important things in life!

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

Auto Mail Sender - Auto Mail Sender Standard Edition official page. A powerful and easy-to-use eMail scheduler.

SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.