VPOP3 has a security feature which will lock out an account after it has had several failed login attempts. This feature is to make it harder for an attacker to use a brute force or dictionary password attack on an account.
The default settings are that after more than 3 attempts to log into an account with an incorrect password, VPOP3 will lock the account for 30 minutes. You can change these settings on the Settings → Security Settings page in the VPOP3 settings.
To unlock an account, you can log into an administrator's account, go to the Users list in the settings, edit the user in question (which will have a padlock icon next to it) and clear the Account Locked Out box on the settings, then press Submit.
If you can't get into the VPOP3 settings because it's the administrator's account which is locked out, there are three options:
The SECURITY.LOG file in the VPOP3 folder on the hard disk will record all failed login attempts, and the IP address where the login attempt came from.