If you ever fail to create a account with Thunderbird, because autoconfiguration does not let you (see https://bugzilla.mozilla.org/show_bug.cgi?id=647761 for example), then do this:
Check File->Offline->Work Offline
and then try to create the account - and now you have the chance to create the account without validation.
Check File->Offline->Work Offline
and then try to create the account - and now you have the chance to create the account without validation.