Right Inbox - Schedule Emails in Gmail To Be Sent Later
Recurrr - Send recurring emails
EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.
FollowUpThen - The simplest way to schedule an email reminder
SendLater - Email scheduler for Mac Mail.
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.