Samsung Flow - Android and Windows Apps.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Recurrr - Send recurring emails
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.
FollowUpThen - The simplest way to schedule an email reminder
Samsung Email - Samsung Email app enables users to receive, manage, and send all the important emails from as many email accounts as they want.