Some of our customers use Google Apps for their email, and may encounter this problem when using a desktop email client.
When accessing your Google Apps email via IMAP by using any email client, such as Outlook, Mail, iPhone, Apple Mail or Thunderbird, the following error message may appear, requesting you to manually input your username and password again in order to authenticate with Google’s GoogleMail IMAP server again:
“Your IMAP server wishes to alert you to the following: Invalid credentials (Failure)”
The error appears to be a way for Google Apps to protect against robots or automated scripts, and also to stop and halt abnormal bandwidth utilization (upload and download) or abusing account activity such as using Google Apps as a storage space drive or transferring another mail account to Google Apps.
Another reason for the invalid credentials or web login requires error to happen is having your mail checked too frequently. Gmail Help recommends setting your mail client to check for new messages no more than once every 10 minutes.
If you’re positive that you’re using a correct username (with @yourdomain.com for Google Apps) and the correct password, and you’re not checking email in your Inbox too frequently, then you may need to clear and unlock the CAPTCHA.
To unlock CAPTCHA for a Google Apps email account, go to the following URL:
https://www.google.com/a/yourdomain.com/UnlockCaptcha (replace “yourdomain.com” with your own domain name)
Type in your username for Google Apps, then type in your password, and fill in the correct CAPTCHA validation. Click the “Unlock” button to fix the IMAP failure error.
If you still have problems after clearing the CAPTCHA, try to use an alternative IMAP server instead, which is imap.googlemail.com, on SSL port 993.