Right Inbox - Schedule Emails in Gmail To Be Sent Later
SendInBlue - Innovative online Email Marketing solution to manage your contacts, create & send your newsletters and track your results. More than 80 000 clients. Best prices and attractive features.
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
FollowUpThen - The simplest way to schedule an email reminder
SendX.io - SendX (https://sendx.io) provides remarketing and retargeting tools for web businesses. You can use popups, landing pages, email, web push notification, automation workflows and FB / Google retargeting to drive more revenue.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.