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.
SendLater - Email scheduler for Mac Mail.
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
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
SndLatr - SndLatr is a Gmail plugin that makes it easy to schedule emails to be sent automatically at a later time.