Fractal is another cool tool that helps email designers check their html code. Just paste in your code and it validates it across 24 desktop, web and mobile email clients. It couldn’t be easier to use. Check it out at: http://getfractal.com/

This is a helpful resource for testing web pages in multiple browser environments. https://browserlab.adobe.com/en-us/index.html

For a monthly fee, access to this system allows you to test emails in multiple email systems. http://www.emailonacid.com/