(a) mind-blowingly huge; (b) totally accustomed to paying serious money for software; (c) almost universally dependent on the Microsoft stack for supporting end users, and that means Outlook for optimal integration with Exchange, Active Directory, etc.; (d) totally addicted to e-mail.
Admittedly, it's also (e) resistant to change and chock-full of drones content with maintaining the status quo. Still, if the Xobnis are persistent, if they keep working on this thing, they'll have a gold mine on their hands.
I still think a cache network would be superior, but Akamai isn't going away any time soon.
Similarly, I think you're in for a surprise as to how slowly change spreads. Corporate environments will likely use Outlook way past its shelf life.
For me that says it all. Maybe it'll be useful in corporate environments that force people to use outlook, but beyond that I'm not so sure it has much of a shelf life.