Services for checking look of web pages and emails
I've used services that do this for web pages, but emails have always been a problem.
The test version, which I'm using at present is giving me Outlook and Google. It shows the mail as it look both unopened and opened. The paid version covers a lot more. With Outlook at maybe 43%, Live/Hotmail at 17%, Yahoo at 13% and Google at 5%, that's coverage of nearly half of the mail reading that I'm getting.
To me that's really impressive.
See http://litmusapp.com/ for details.
They have an analysis of what mail readers people are using, where I got those stats above, see http://j.mp/dbtAfv for those details. They run a sister product called Fingerprint that can do that for you.
If like me you are passionate about doing the web right you might like to have a look at that. (Be aware. They throw in some validation of page content with the web page tester which you get with the email tester. It's W3C engine based and in not as good as CSE HtmlValidator.)