What's interesting is if you look at gmail headers, they do indeed route via public IPv6 back to themselves for gmail-to-gmail transactions.
Or at least they log the ipv6 route within their own datacenter, or between datacenters.
So that is up for argument if it is traveling via their own intranet or "the internet".
(at least google uses AES TLS, yahoo still uses RC4)
This, of course, requires that everyone use the hosted email client on that server, but it's worth remembering.
Personally, I (and everyone at rsync.net) just use (al)pine, so the hosted email client is very simple (and fast and efficient).