One of my back-burner ideas is speeding up email forwarding. Most email forwarders (sendmail, etc.) accept emails, put them in a queue, and then later spool them out to the destination. This adds a minute or so of latency. It's done this way for historical reasons. In the early days, the destination mail agent might be down, or the mail transfer might be over some polled protocol like UUCP.
That's dead. Today, if the destination mail agent exists, it's probably up and immediately reachable via a fast connection. So a modern mail fowarder should accept the incoming email via SMTP, and then, while holding the incoming connection open, send the email on to the destination mail agent. Any problems are immediately reported to the sender via SMTP status code.
This not only speeds things up a bit, it eliminates "bounce messages" generated between mail agents. Problem reports come back immediately, as SMTP errors. There's a series of open TCP connections from sender to the receiver's IMAP server. From the IMAP server to the final destination, today you usually have some kind of push notification.
So you get the effect of instant messaging, using existing email protocols.
This also eliminates "joe jobs", where impersonation generates vast numbers of bounce messages. The spammer just gets lots of SMTP errors, which never bother anybody else.