In order to use Yahoo email over POP3/SMTP, you need to install YPOPs. After you install YPOPs, you need to configure the dispatcher.
After YPOPs is installed, configure the dispatcher as follows:
Click the dispatcher icon on the system tray and select the Collaboration Wizard
Select "Using email for collaboration" if it isn't already selected
Select "I have a POP3/SMTP account for use by Code Co-op"
Known Issues: Yahoo released their beta which doesn't currently work with YPOPs. If you are using the new beta, you will need to revert back to the previous version of Yahoo. You do this by logging on to Yahoo from your web browser. Click "Switch back" at the top of the page.