This is pretty common. I field a half dozen requests like this a year for Twiddla. Very few of them pan out to actual deals.
A good plan to avoid wasting your time is to give them a ballpark figure up front for what it will cost to make this happen. That figure should not be less than six figures if it involves any significant effort on your part.
If you've already packaged everything up, and it really is as simple as delivering a VM image, you might only quote them 20X - 100X the retail price of your most expensive subscription plan. Naturally, this will be an annual contract, with the first year paid in advance and a fair amount of lead time.
Keep in mind that your email outlining this will typically be the last contact between you and the customer, since they're often simply middle manager types with an expectation that the price will be the same as your "Big Customer" subscription level, except that it will somehow magically run behind their firewall. Your mail will simultaneously give them a heart attack and correct their understanding of Enterprise licensing. If they do engage after that, you can start your Enterprise Sales cycle.
Good luck.