I think you are about to invent the mail server with delayed send feature. Because how else do you call a permanently on, always connected machine that send emails at set dates?
extrowerk is right, this “feature” is useless on a laptop mail client, and possibly dangerous as it confuses the user. I don’t want such things failing before me. It may make sense on a mobile phone, which is more likely to be permanently on and connected these days. But certainly not on a desktop or laptop system.
The insistance on doing things like this is why open source software never takes off. We develop a tolerance for such quirks, because we are technical users and understand the limitations. But for a regular user without this background, we should take all the efforts to make better protocols and bring such functionality in.
If it can’t be done through the usual mail protocols, let’s switch to something else. XMPP, maybe?