Salesforce Error Notification – “SOAP response was a nack”

Can you please help us to resolve the error I banging my head from two weeks not able to find the root cause and we have outbound message it will fire every one hour if criteria meets. the problem was outbound message not sending the Ack message .when we check the outbound monitoring status Delivery failure status displaying as “Soap resoponse was Nack”.does have any solutions to resolve this