Yahoo Mail POP
In your Yahoo mail mailbox, click "Mail Upgrades" and then on "POP Access and Forwarding." It costs $19.95 per year, and when you register, they will give you the POP server address (probably mail.yahoo.com, but I'm not sure).
And if you aren't familiar with POP accounts already:
Once you are registered ,go into Mail Preferences, and click "Accounts" Click the "Add Account" button. Now select "POP" from the "Account Type" list, type a description, your yahoo email address, name, POP server (that they gave you when you registered), and your yahoo username and password.
Now Mail will automatically include your Yahoo mail when you check mail.