Increased focus on security and privacy has led many common e-mail clients to block HTML image loading. This now affects many of the biggest providers and the most popular software, including AOL, Hotmail, Yahoo!, Gmail, Outlook 2003, and Thunderbird. Though HTML isn't always blocked or prevented from displaying, it can greatly affect how your message appears.

Worse, these systems don't work exactly the same. Most leave a blank space where an image would be. In Yahoo!, the space is gray; Gmail displays a broken image icon. Some, such as Gmail and Thunderbird, will also display alternate text if it's provided. Others don't. AOL not only blocks image loading but also disables hypertext links.

Despite all this variability, you can optimize your recipients viewing experience.

Get the full story at ClickZ