Prepare the mail And schedule the sending like in Outlook or like the plug in "boomerang" for gmail.
Keith Casey commented
Btw, this other request is the same and has more traction:
I have wanted this feature too in fact I've started to use Sanebox (no I'm not affiliated) which seems to do it
Using the mailbox and even accompli ideology. It is grey to move emails to time specific areas and have them pop back in to your inbox when you want to deal with them.