You know one thing, You can schedule your email also in Gmail through Google sheet(Google Docs). Suppose you want to schedule your birthday wishes email to your loving friends. You just make a draft and save it. Go to your Google Sheet and authorized yourself. Then import all draft message from Gmail. When you click on the import button, it will automatically fetch your all draft messages of your Gmail id. Now you can set schedule time and save. All done. Now you can test.
Here is a wonderful video tutorial by Amit Agarwal which I would like to share with you.
Hope this is helpful for you.