If you use Google Apps with an IMAP client such as Apple Mail, Entourage, or Outlook, periodically you may simply stop being able to log into the incoming mail server, even after you’ve double and triple checked the settings. If you have Google Apps standard (free), or Gmail, you may simply be required to log in on the web site once in a while. But if that doesn’t work, you may need to go through a special security login that Google requires. To do this, go to the following URL (substituting your domain as appropriate if it’s Google Apps):

https://www.google.com/a/yourdomain.com/UnlockCaptcha

For gmail.com addresses, it’s:

https://www.google.com/accounts/DisplayUnlockCaptcha