This can be caused by a number of things, the main one is trying to access your mailbox with the wrong username and/or password.

Your username (or Account Name) is the name before the @ in your email address.
eg. for info@yourdomain.com.au the username is info

The password in what you specified for the mailbox in your Hosting Control Panel.

If that doesn't work, try changing the mailname password in Hosting Control Panel, then update the new password for your mailbox in your email program.

If all else fails, simply remove and then recreate the mailname in your Hosting Control Panel.