SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Recurrr - Send recurring emails
Geary - Geary is an email application built for GNOME 3.
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.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.