Slightly off-topic: with email delivery times being almost instantaneous these days, wouldn't it possible to write an email client (or app) that provides, for all practical purposes, an IM system based on SMTP and IMAP?
Essentially, conventions like:
1. A set of recipients is a "group"
2. Replies are never quoted
3. The client always shows the full email thread (the "conversation")
4. The Subject line is a magic string to allow filtering on these IM emails
The specialized client would provide a very Whatsapp-like interface, while a normal email client could be used in a pinch as well.
Am I overlooking something obvious?