Recurrr - Send recurring emails
Samsung Flow - Android and Windows Apps.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Huawei Mobile Services - Huawei Mobile Services app comes up with features to help you in accessing a large number of advanced features such as payments, push notifications, and Huawei ID, etc.
GMass - GMass allows you to easily schedule emails, send out mass emails and mail merge campaigns and more with Gmail! Click here to learn about more features!