If you are using 2005 version it has an option where you have to enable Yahoo mail filtering. Have you browsed through for a selection like that in the options.
I check my yahoo mail through outlook (using POP3). I installed norton antispam, and now outlook will not download my email to my computer. I tried configuring the antispam program and nothing works. I even disable the email scanning portion of the program and it still doesn't work. When I stop the norton antispam, outlook downloads my yahoo email just fine. Anybody know how to configure norton antispam.