Collecting mail from Gmail using POP3
Gmail's POP3 servers are not standard POP3 servers, so you need to set your Gmail settings appropriately, and, preferably, understand how Gmail implements POP3.
In your Gmail account, go to the account settings
Then go to the Forwarding and POP/IMAP tab and look at the POP download section
Check the option which says "Enable POP for all mail (even mail that's already been downloaded)", and press the Save Changes button at the bottom of the settings.
See these Gmail ...