"Relay Access Denied" on our application sign up account (which has not been touched)

"Relay Access Denied" on our application sign up account (which has not been touched)

We're getting  "Relay Access Denied" bounce backs to our "no-reply" email account which sends users activation emails. 

There have been no configuration changes to the application server or network infrastructure.

Can a member of Zoho's support team assist please?