Interested to know if everyone who has this problem is also receiving mail from a Microsoft Exchange server. I have also had this problem on several different platforms from XP to Vista, from Outlook XP to Outlook 2007. I thought it was Cloudmark Server causing it, but I also have the problem on servers without Cloudmark. However, I have only had the problem with Exchange server.
When I log into to Outlook Web Access the formatting is stripped as well. Leads me to believe that the Outlook desktop application is not the source.