Samsung Flow - Android and Windows Apps.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.
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.
SendLater - Email scheduler for Mac Mail.
Samsung Email - Samsung Email app enables users to receive, manage, and send all the important emails from as many email accounts as they want.