Mac: Sometimes you need to send an email at a specific time. Maybe it's an anniversary or just a reminder. Chrome extensions like SndLater let you do this with Gmail. If you have a Mac, Automator lets you schedule mail through Apple's default Mail app.
Apple's built-in email program scored some enhancements with Yosemite. If your Mac supports Handoff, you might be using Apple Mail more often than you used to. But even if you aren't a regular Apple Mail user, as long as the account is set up, you can schedule emails with Automator, no extra software required.
Instead of creating the message within Mail, use Automator's "New Mail Message" action to create a stand-alone app that sends the email at a certain time. Then, to schedule the email, go into your Mac's Calendar app (fomerly known as iCal) and set your Automator app to open at the time you want to to send an email.
Check out the link for the details.
How To Schedule Email On Your Mac [MakeUseOf]