yahoo mail is not accessible only yahoo mail plus is. so if you setup yahoo mail, settings used are for pop3 and wont work for yahoo mail users.

setup nokia messaging at email.nokia.com, login and add emails accounts you want access e.g gmail, yahoo, hotmail etc. once you have added accounts on nokia messaging website. Setup nokia messaging account on n900. n900 will automatically setup all the accounts you setup earlier on email.nokia.com

done!