This has been something that I've looked at a few times over the years. There have been a number of FLOSS and commercial projects that have tried to do this (and all that I'm aware of have sputtered out). A defunct product called "Mailonator" (http://web.archive.org/web/20080611060558/http://www.mailona...) did exactly what you're talking about.
We did this at my previous workplace. We stripped emails, encrypted them, and sent an email with the link to the files. Only an approved app was then able to open such files.
