Yahoo Mail supports POP and also IMAP access via third-party email clients like Thunderbird, however, what if all that you want is that all your incoming mail to get delivered to another account? Easy. Go into the settings (bang the gear icon on the upper right of your desktop in Yahoo Mail), click Accounts, then the Edit button.
Click the radio button on Forward, and insert the email where you’d like messages to end up. You also get the options to Store and Forward (which keeps messages/notes on Yahoo Mail even after forwarded), to Forward Only, or Store and Forward and Mark as Read—so you don’t have to deal with those messages anymore when you visit Yahoo Mail.