Ben Chestnut wrote a great article aimed at web designers that are making "stupid" mistakes when they try to create HTML emails. "It's one thing if it's some small business owner who goofed on a CSS tag. That's forgivable. But it's astounding how many professional web designers (and programmers!) make silly mistakes in their HTML email campaigns." he says.
Mistake #1: Not designing for preview panes
Mistake #2: Assuming images will work
Mistake #3: Too many images, not enough text
Mistake #4: Not testing in different email programs
Mistake #5: Neglecting your footer
Mistake #6: Too fancy-schmancy
Read the article here and pay special attention to the tips Ben gives to avoid each of these mistakes.
Thanks Jeanniey for spotting this one, I totally missed it! :)