Still sending HTML emails? If you are, make sure to use images correctly. Many email clients block images by default and some don’t even show the alt text if you included it.
For some great tips on making the most of your images in HTML email, go read David Greiner’s article called Email Design Guidelines for 2006 | Articles/Tips on the Campaign Monitor Blog.
Just like with HTML-based design for a web site, it is important to know your audience when sending out HTML emails. David’s article will help you get there.
[UPDATE: fixed typographical error.]