[zefhous]

It would be great if this app provided some other functionality that helps with problems like that.

For instance, MailChimp takes CSS styles and modifies your HTML by placing your CSS into inline style tags. This is great, because you can have cleaner markup but still support clients that don't apply CSS well.

[revetkn]

Litmus will take an HTML email and run it through all sorts of mail clients, providing screenshots of how each one renders: http://litmusapp.com

(I'm not affiliated with them in any way, just found the app to be very useful while working on a recent project)