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?
Not like you have more options for small embedded code for GUI in c++ (if i had to, i would choose FLTK with some more modern theme). That would have made it portable to other systems (despite looking dated, i know). Or go a little more advanced and get wxWidgets, for example.
The butter for windows software is now at C#/.Net family and usage of the legacy winforms or the newest version of XAML (whatever is called now).