SMTP Connection Failure

Hello
We are running Exchange 2003 with service pack2.
Recently we are not getting emails from particular companies.
Even Yahoo. This is the error message from yahoo:
reason: 550 5.7.1 Requested action not taken: message refused) —– Transcript of session follows —– … while talking to natform.com.: >>> DATA <<< 550 5.7.1 Requested action not taken: message refused 554 5.0.0 Service unavailable . Let me know if you need more info.