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.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
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.
Prospect.io - Prospecting, like a boss. Find any email, reach out, measure everything and integrate with your CRM.