An email program is rejecting the username/password, or giving an "Invalid Credentials" error when checking for messages.
The username in the email account settings may not contain the full email address, or if this is a Google-based email account, the account may have been locked due to suspicious activity.
If this is a Google-based mailbox, try the following:
- The mailbox may be locked due to suspicious activity. This can be caused by setting an email program to check mail more than once every 10 minutes, too many failed logins, or sending large amounts of email in a short period of time. Go to Google Captcha and follow the on-screen instructions to unlock your account.