To me it seems the way they have Thunderbird set up to add new accounts seems not very user friendly.
Since I have no idea wht OPTUS is, if you set Thunderbird up manually, what are you picking for the server. You need to make sure the type of server is correct, pop3 or whatever, and the other information is correct. It asks you for a password it seems like twice. During the setup and the at first account access. I may be wrong about that, but it just seems that way.