In the menus go to "Tools > Account Settings" and select Server Settings. Change your Username - it doesn't matter what you change it to, add an "x" to the end - then click OK. You'll get a warning about changing email address just click OK. Now click the "Get Mail" icon and you will be prompted for your Password. Click "Cancel".
Go back in and change your Username to the *correct* one again. Now when you click "Get mail" and are prompted for your Password, you can enter the correct Password.