Ever noticed how most HTML emails fall into the range of 500-650 pixels wide? The main reason is that the message windows of popular web-based email services such as Yahoo! Mail, Hotmail and Gmail will typically present 500-650 pixels (according to the user’s screen resolution settings).
In simple terms this means that HTML messages exceeding this range require the recipient to scroll horizontally in order to view the email in its entirety. Horizontal scrolling may look cool on a fancily designed website, but should be avoided at all costs when it comes to emails. Users giving your email a quick glance may get cut off from key content and crucial images. In addition to irritating the reader, you risk losing a transaction or a subscriber.