Another Gmail quirk - I don't know why it happened to me, but if you get this error
This is what their official help suggests
"We suggest making some adjustments in your mail client's settings so you aren't always prompted to enter your username and password. Please check the following settings in your mail client:
- Make sure that you entered @gmail.com after your username. For example: email@example.com
- Re-enter your password to ensure that it's correct. Keep in mind that passwords are case-sensitive.
- Make sure your mail client isn't set to check for new mail too often. If your mail client checks for new messages more than once every 10 minutes, your client might repeatedly request your username and password.
- Disable all POP clients you're using to read Gmail messages. (If you use one at work and one at home, please disable both.)
- From the computer on which your POP logins are failing, visit http://www.google.com/accounts/DisplayUnlockCaptcha
- Enter your Gmail username and password, and the letters in the distorted picture.
- Once you have successfully logged in, restart your mail client and try to download your mail."
Worked for me :)