First, make sure that you have already created this POP email account through your online cPanel control panel. Please see the Control Panel User Guide to learn how to do this. Also, be sure that you are using the same user ID and password that you set up through the cPanel for that particular email account, and keep in mind that both the user ID and password are case-sensitive.
Be very careful about entering incorrect login details too many times. If the firewall detects multiple failed login attempts, it may trigger security protection and block you from accessing the website.