Mensagem de Retorno!!!

Pessoal, estou com problemas. quando envio mensagem ela vai sem problemas, mas quando envio de outro lugar como hotmail,bol, .... e ele nao recebe, e retorna uma mensagem como esta abaixo:

This is the Postfix program at host master.termonorte.com.br.

I'm sorry to have to inform you that your message could not be
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The Postfix program

<[email protected]>: [brav.brav.com.br]: Name or service not known

Final-Recipient: rfc822; [email protected]
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; [brav.brav.com.br]: Name or service not known