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.
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.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
NudgeMail - Write or Forward any email to NudgeMail.