Samsung Flow - Android and Windows Apps.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
FollowUpThen - The simplest way to schedule an email reminder
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.
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
Samsung Email - Samsung Email app enables users to receive, manage, and send all the important emails from as many email accounts as they want.