This is as much for my own reference, but here are some tools to test and refactor HTML templates such that they’re more widely compatible with the plethora of mail clients out there in the wild:
Litmus Email Testing – freemium model, multiple client testing areas, spam-like content detection. Looks good!
MailChimp CSS Inliner – converts styles to inline, which is more cross-client compatible. Smashing Magazine says “Code Like It’s 1999 (Literally) and Use Inline CSS”