"M$ HTML email" is not a W3C standard !
In case I didn't mention it already "M$ HTML email" is not a W3C standard !
AND M$ have changed it again in their latest email client ! BUT they have been kind enough to create a special "M$ HTML email" validation program you can use to test your non-standard emails !
http://msdn2.microsoft.com/en-us/library/aa338201.aspx
http://msdn2.microsoft.com/en-us/library/aa338200.aspx
Join the VOX W3C Web Standards group today !