[Animats]

This could work really well with minor fixes. There are additional "enhanced status codes" available for SMTP that aren't used much. IMAP servers could return a new status such as "2.2.5", with meaning "Delivered, recipient notified". That would indicate an active NOTIFY connection between IMAP client and server. That means the client is listening right now and will display the message.

With "forward while connection is open" mail forwarders, that code would be passed back to the originating SMTP sender, which can then display an indicator in the bubble that the message has made it all the way to the recipient's device.

Now it looks just like messaging, but works over email infrastructure.