AT&T WorldNet service has banned the sending of anonymous email or posting anonymously.
From the "AT&T WorldNet Service Operating Policies":
(i) Members may not post or transmit any message anonymously or under a false name. Members may not permit any other person (other than an agent acting on Member's behalf and subject to Member's supervision) to access the Service Member's account for any purpose. The no anon rule even beat the no indecency rule, which is second: (ii) Members may not post or transmit any message which is libelous, defamatory or which discloses private or personal matters concerning any person. Members may not post or transmit any message, data, image or program which is indecent, obscene or pornographic. http://www.worldnet.att.net/care/terms/#oppol