To quote from
Yahoo help pages:
"
Why am I having difficulty sending Yahoo! Mail messages from an email client?
For security reasons, we require that you log in to our POP servers before you may send mail from an email client. To do this, your email client will need to download new messages before attempting to send messages.
If your email client sends mail first when you click "Send/Receive," you may need to click "Send/Receive" twice. If you can't send mail after several tries, move messages out of the Outbox, attempt to receive with an empty Outbox, and then try to send your message again.
If you try both of the above steps and continue to have problems, please make sure that you have entered your Yahoo! Mail Address as the "From" address in your email client. You will not be able to send mail if you have entered another address.
Finally, you may also wish to call your ISP to ask if they block outgoing mail servers."
~Ladi