Here are some possible reasons your external mail may not be working:

• Make sure you entered the correct information when you set up your external mail settings. Double-check: Did you enter the correct mail server address? The correct port number? Correct user name and password? We don't have any way of checking if you entered valid information.
  Unfortunately, Yahoo! Mail Support doesn't know and can't provide you with any of this information. For assistance tracking it down, contact your Internet Service Provider (ISP) or Network Administrator.
• Make sure that there are messages waiting on the external mail server. If there aren't any messages, there isn't anything to download.
• The server you're attempting to contact could be behind a firewall. A firewall is a combination of hardware and software products that protect a network's security. Ask your Network Administrator if a firewall is preventing you from accessing external mail.
• There could be problems on your local network or your external mail server. If you recently began having problems (and you haven't changed your settings recently), you might want to contact your ISP or your external mail server's Network Administrator.

Please Note:

• AOL uses a proprietary email system which does not support POP access. This means that you can't retrieve messages from an AOL account through Yahoo! Mail.
• With Yahoo! Mail, you can only retrieve messages from other Yahoo! accounts that are enabled with POP access.