We’ve just come across a real useful tool for those HTML email developers out there, it’s called Fractal. Fractal is a tool developed by Campaign Monitor that validates your HTML and CSS through a number of reputable sources, including the highly regarded Email Standards Project and lets you know which clients your email will and won’t work in. Fractal also flags the parts of your code that are unlikely to work, and gives you suggestions on how to fix the problems.

Check it out: getfractal.com.