SendRecurring VS EmailPlayground

Right Inbox - Schedule Emails in Gmail To Be Sent Later

SendLater - Email scheduler for Mac Mail.

Recurrr - Send recurring emails

FollowUpThen - The simplest way to schedule an email reminder

SndLatr - SndLatr is a Gmail plugin that makes it easy to schedule emails to be sent automatically at a later time.

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.