I am not sure about the US market, but Skyscanner [1] works great in Europe, I wonder how this is different. Besides from being structured as an email, it offers quite similar functionality of finding optimal flights (in some sense of optimal). It may just be me, but I am definitely misunderstanding the inbox metaphor. Why is it an inbox? Why is there emphasis on _one email_? I don't book flights through email -- I go on an airline's website to book.
For now it seems to be always responding with "Hi there, I couldn't find any flights for your request. Please try again.", I could not test it out. Unless this is a demo version, and is working as expected.