If you use Outlook 2007 onwards you may have noticed that HTML emails don't always
display as you've seen them in other email clients or as you'd expected them to
The reason for this is that the rendering engine in Outlook 2007 onwards is
the one from Word which a) isn't fully standards compliant and b) is limited in
it's CSS and HTML support. Quite why Microsoft chose this route is a mystery,
given their definite move towards standards with Expression Web.
Email Standards Project
If you are particularly interested in HTML emails see the
Standards Project which is campaigning to resolve the differences
the exist between email clients in their HTML support. It's like going
back five years with browser standards!
The following articles on SitePoint are worth reading.