This is interesting - I'm assuming that by "Exchange Integration" you're talking about Calendaring, appointments, and so on yes? I don't get the connection between your use of Exchange and your client's use of Exchange.

Put another way - I'm imagining you have a "Add to Outlook" bit of functionality which makes perfect sense. But the format is quite open and it, after all, a text file. How does this jibe with your web offering and your need for Exchange?

The main reason I ask is that Exchange is (generally) seen as not necessary in a startup (company < 20 or so). Typically they'll use Google apps or another hosted service which will accept their domain (Live.com does this as well).

Can you detail more why you needed Exchange with a Web app?