If you want to use Outlook, Outlook Express, Thunderbird or other third party email clients to view your email you have 2 options.
Subscribe to mail plus:
or download and install YPOPS
then follow the directions for your version of outlook here,
If you have trouble with set up, check the YPOPS installation page for alternate set up instructions.
Click on YPOPS link on the left.
Then Click on Configuring email clients towards the bottom.
Choose the appropriate link for your mail client.
I currently utilize version 22.214.171.124 of Thunderbird
and version 0.8.3 of YPOPS (there is a more recent version of YPOPS that you need if you are Yahoo Beta Mail) and have no problems.