It's a message rule in Outlook 2007. Can disable or remove this if you want
to. There is no such default in Outlook 2003

Signature
Robert Sparnaaij [MVP-Outlook]
Coauthor, Configuring Microsoft Outlook 2003
http://www.howto-outlook.com/
Outlook FAQ, HowTo, Downloads, Add-Ins and more
http://www.msoutlook.info/
Real World Questions, Real World Answers
-----
> I seem to remember that there is a setting in Outlook 2007 that allows a
> category assigned by a sender in the sender's email to be preserved in the
> email when received by the recipient. I can't find the setting again. Is
> there a similar setting for Outlook 2003? Can any one help?