Ever notice that most of the HTML emails you receive appear to about the same width? Measured in pixels, most HTMLs fall into the range of 500-600. The early email clients were developed pre-HTML and thus were not designed to render wide Web page-like emails. More recently, the message windows of popular Web-based email services such as Yahoo! Mail, Hotmail and Gmail will typically present 500-600 pixels - varying based on the user's screen resolution settings.
In simple terms this means that your HTML messages that are wider than this range will require the recipient to scroll horizontally to view the entire width of the email. While forcing a user to scroll horizontally might be OK on your Web site, it should be avoided with your emails. Users may only give your email a quick glance and with key content and images getting cut off, you risk losing a transaction or reader, and at minimum being an irritant.