Yahoo has announced the availability of the Yahoo Mail Web Service. This new offering will allow developers to use API protocols, via SOAP or JSON-RPC, to connect to the Yahoo mail platform.
This enables 3rd party authorized access to list email messages, compose new message, send email, read email and so on, all from outside of the Yahoo Mail interface.
I believe this is only available to Yahoo Mail Premium users, and Yahoo is offering $10.00 commission for each person you sign up with the premium mail service.
This comes after Yahoo announced unlimited email storage yesterday. There is more coverage at Techmeme.com