Samsung Flow - Android and Windows Apps.
SendLater - Email scheduler for Mac Mail.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
The Top Inbox - The Top Inbox for Gmail helps to schedule emails to be sent later, set reminders in the inbox and follow up with email sequences.
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.
EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.