Same here. You're welcome to make a Mac-exclusive mail client, but you're gonna have a damn hard time making anyone care about it when only 7% of the active computer-using market is capable of installing your app...
Love it or lump it, that's where the users are. Limiting your userbase inherently gimps the value proposition of your application, which is especially an issue when your program is something as simple as an email client.
Why would you need to make anyone outside of Mac/iOS world care about it in order for your app to succeed?