I keep thinking that Email would be a pretty natural extension process with the workers model in general... if they offered workers that could handle a tcp connection as stdin/out from the application perspective. Especially in concert with D1, R2 and other services.

I think the biggest issues would come down to server-side search functionality though. For very basic services, and even most of common IMAP/JMAP, it could be pretty great. Working on an a major email platform is something I've really wanted to do for a while now. (cloudflare, call me)