Yahoo Mail "regect" Questions

I can't register on yahoo mail,server always regected my country-serbia,and every post code.Why???????? try to leave it blank. frank malawi dawdw

e-mail error cation could not be sent because the server regected the senders E-mail address acount pop.att.yahoo.comsever"smtp.att.yahoo:,protocol,"smt,server resourse:530 authentication required ...