Don't use <p>, <div>, <ul>, or <li> tags. Stick to tables when building your HTML for email clients. Other tags don't have universal support across all email clients, so it's best to keep it simple to ensure that your email performs well for everyone.
Don't use Dreamweaver to edit or create your HTML. Dreamweaver is not a program that was designed for editing or creating code for emalls. It will create code that doesn't follow the best practices isted here.
Don't use background images. They are otten not displayed
Don't use absolute positioning.
Don't position images using X-Y coordinates.
Don't use the image "low source" attribute (lowsrc).
Don't use the image "roll-over" attribute (hsrc)
Don't use the image map "shape" attribute: image maps should not be used at all due to inconsistent support.