If you have a Yahoo! mail account set up in Postbox and you cannot get new mail because you receive a message saying "The RETR command did not succeed. Error retrieving a message. Mail server pop.att.yahoo.com responded: problem retrieving message." then try the following:
- Quit postbox
- Log into the Yahoo webmail client
- Empty your Spam Folder
- Restart Postbox and attempt to get new mail