My Dream Webmail app is the one I can almost be fooled into thinking isn't (a web app). Some recommendations I would make in this regard.

• Sieve like filtering. If you support multiple folders then filtering mail on delivery rather than after discovering 500 new messages in INBOX would be great.
• Try setting up a couple of accounts in Outlook, Evolution, Thunderbird and perhaps Kontact. Observe the things that you really hate about these apps - try to avoid them or at least find a better way to achieve them.

I think the point I am making is don't limit your goals too early by focussing on 'WebMail'. You want to write an MUA from scratch , it just so happens the user interface is not Curses or gtk or insert witticism.