The biggest problem with (human-written) email is that it's so poorly written. 20 action points, mixed with a bit of smalltalk, and lots of tangential information, all on a reply to a totally irrelevant email. The kind of people that send these emails either wouldn't use an 'expires' header, or would set the expires header to be '5 minutes from now'. They're the same people that have 'top priority' as the default setting on every Outlook email.

However, there are ways of adding functionality to email clients without every single email client implementing it. Think YouTube previews in Gmail, and all the other custom headers on Exchange servers.