By default, when Outlook connects over a POP connection it will download your email and then remove them from the server (in this case Yahoo). You will want to configure your Oulook to leave the messages on the server. This can be done as follows:

1. Click the File menu.
2. Click the Account Settings button, and then click Account Settings.
3. Double-Click on the POP/SMTP account in the Email box.
4. Click on the More Settings button.
5. Click on the Advanced tab.
6. Check the box to "Leave a copy of messages on the server".
7. Click OK.
8. Click Next and Finish.

With these steps, Outlook should leave new email in Yahoo after it retrieves it.

-SW