SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
Recurrr - Send recurring emails
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
FollowUpThen - The simplest way to schedule an email reminder
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.