It works on the iPhone because Yahoo have setup a special way of working with the iPhone called 'Push' email, it is the way you get your emails on the iPhone as soon as they arrive. This in effect means that on the iPhone Yahoo are offering an IMAP mail service. Unfortunately Yahoo do not offer an IMAP mail service for computers, they only offer POP email. This is not a failing of the computer, it is down to the service that Yahoo provide.