Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
Yesware - Sell smarter with email tracking and more
FollowUpThen - The simplest way to schedule an email reminder
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Mixmax - At Mixmax, our mission is to change how the world communicates. We’re an email productivity platform that helps sales, customer success, and recruiting teams do their jobs better.