The server response was "The server response was: 5.7.1 : Client host rejected: Access denied

We are facing an issue with the outbound mail authentication. We have entered all the credentials and after saving that, If we hit on the Test connection button we are getting a pop up window showing " Failed to send Message <from Mail Id> to <To Mail Id> ."  And we perform some Add assignment action to know about this issue, there we are getting this response : "The server response was: 5.7.1 <unknown>: Client host rejected: Access denied ". Kindly let us know the reason behind this issue. For reference, i have attached those error screenshots. Have a look at them .

Thanks,

Murali