Any one who has ever designed and built for Email knows how frustrating it can be. It can some times be worse than dealing with legacy browsers and all their quirks.
Email has come a long way in the last few years but there are still no standards, although the people at http://www.email-standards.org are trying.
Many great tools exist like Litmus.
A perfect companion to Litmus would be Fractal, It’s a code QA test rather than a visual validation.