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.
Affinity - Relationship Intelligence, Reimagined
Rebump - Automatic follow up emails. Keeps on bumping until you get a reply.