Try this:

In your Thunderbird program, look up the Account Settings for the mail user with the login problem. You can do this with a right-click on the account and then choosing Properties from the context menu.

Under Server Settings, make sure the User Name field does not have a domain. For example, it should read storekeeper instead of [email protected].

PS: "www.localhost.com" won't work, only "localhost" or "www.example.com" will work.