I just discovered that it is possible to tag mails with categories in microsoft outlook 2003. Once you have done that, it’s easy to display mail in a folder by categories. It is a more flexible way to organize mail than folder, because you can attach a mail to several categories whereas it can only be located in one folder. The next step is to create a search folder containing all your mails and to display this folder by categories. So the Outlook categories works like tagging in social bookmarking site like del.icio.us.