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

Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity

Right Inbox - Schedule Emails in Gmail To Be Sent Later

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