Why does Outlook 2010 render so differently to Outlook 2003?
Outlook 2010 actually relies on the Microsoft Word rendering engine, whereas Outlook 2003 and earlier versions have used Internet Explorer.
Microsoft Word only supports a smaller subset of the rendering capabilities of Internet Explorer, and can be quite difficult to work with.
We recommend checking out this list of supported HTML elements, attributes and CSS rules as a starting point.
How important is it that my Email renders correctly in Outlook 2010?
Based on our aggregated Email Analytics data, Outlook 2010 represents just 1.09% of global Email client market share. That figure is increasing each month though, as more users upgrade from earlier versions of Outlook.