Here is another one: Sell a a nicer "native" client, either by wrapping the website in a custom browser like http://mailplaneapp.com/ http://mailplaneapp.com/,

Here is another one: Sell a a nicer "native" client, either by wrapping the website in a custom browser like http://mailplaneapp.com/ http://mailplaneapp.com/, or make a version that looks as close as possible to Exchange.