Vždyť ti to tam píše:
http://worldnet.att.net/general-info/bls_info/block _enduser.html
521=
521 <0.0.0.0> is blocked
The sending system has been identified as a source of spam.
Please see http://www.att.net/general-info/bls_info/521.html for more infomation.
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Pošli to z jiné mailové adresy.