Attempts to download or send mail are failing. You might get a message
about how your credentials aren't valid, or your email client might
keep asking you for your password, over and over. You've already
checked that POP or IMAP is enabled in your Gmail Settings, your email
client is completely updated, and you're certain that your email
client's configuration matches the instructions in the Gmail Help
Center, down to the last screenshot. Still, you're stuck.

Luckily, you might be able to resolve this problem by clearing the
CAPTCHA. To clear the CAPTCHA, follow these steps:

1. Disable all mail clients you're using to read mail. (If you use one
at work and one at home, please disable both.)

2. From the computer or device on which your logins are failing, clear
the CAPTCHA.

--- For Gmail users, go here: http://www.google.com/accounts/DisplayUnlockCaptcha

--- For Google Apps users, use this address:
https://www.google.com/a/[YOURDOMAIN.COM]/UnlockCaptcha

*Be sure to replace [YOURDOMAIN.COM] with your actual domain name.
Include everything after the "@" in your email address. For example,
if your email address is "yourname @ wildblue.net," you will put
"wildblue.net" into the URL: https://www.google.com/a/wildblue.net/UnlockCaptcha

3. Enter your email username and password, and the letters in the
distorted picture.

4. Once you have successfully logged in, restart your mail client and
try to download your mail.