YPOPs! is a free open source software that provides POP3 and SMTP access to Yahoo! Mail. You can use your favourite email client, be it Outlook, Thunderbird, or whatever else you like and connect to Yahoo! Mail seamlessly.
This application is more like a gateway. It provides a POP3 server interface at one end to talk to email clients and an HTTP client (browser) interface at the other which allows it to talk to Yahoo! The same concept holds good for SMTP as well.
Download: YPOPs! 0.9.7.3