We encountered a pretty interesting problem recently, where the POP3 or IMAP does not work for just one particular google app mail account in Outlook.

It was baffling because POP3 or IMAP works for every other mail accounts in the domain except for just this this so it couldn't be a DNS problem. The port and securities settings were exactly identical as well so the fault must lie somewhere with the particular account.

Turns out that Google perceived the default password to be too weak and has added an additional captcha code feature to strengthen the security. While you will encounter the captcha code automatically if you were to use a web login, you're stuck if you're using POP3 / IMAP.

To resolve this, go to: https://www.google.com/a/YOURDOMAIN.COM/UnlockCaptcha (replace YOURDOMAIN.COM with your actual google apps domain).

Key in the username/password and matching characters to the squiggly words in the picture (this is known as the "captcha code")

Once successful, you should see a "Captcha succesfully unlocked" message

Try sending and receiving from your POP3 / IMAP now.

If it resolves the problem, the next step would be to change to a stronger password (more than 8 characters, combination of alphanumeric, etc) or Google may lock the account again within 24 hours :)