Are you sure you have POP3 access enabled in your Virgin Media account? With some email providers you have to explicitly enable POP3 access. Or, are you trying to send and receive from your ntlworld mailbox but have your email address set up to look like @virginmedia.com? If this is the case, it looks like YAM is setting the POP3 address automatically for you, and you should go to the account in the TCP/IP section of your preferences to set it back to the correct ntlworld.com address...
