Certain Yahoo! email accounts, mostly the ones in North America cannot be configured with the Apple Mail application unless the owner of that account has a paid subscription with Yahoo! specifically to enable POP access. To my understanding the reason behind this is because Yahoo! makes money when people check their email through the webmail access point, also exposing themselves to all the advertisements scattered throughout the interface. I believe its the same policy for Hotmail/MSN/Windows Live. I am not certain that the UK accounts are subject to this same subscription fee because some other Yahoo! accounts, such as the ones in Southeast Asia and elsewhere can be setup without such a fee.