3314505 - Emails are failing with error XS855: No delivery as sender or receiver address invalid -Sender address rejected

Emails being sent from the system are failing with error message XS855- "No delivery as sender or receiver address is invalid", the long text of the error shows the following:

No delivery as sender or receiver address is invalid

Message no. XS855

Diagnosis

The message was processed correctly in the SAP system. However, a subsequent system has classified the sender or receiver address (or both) as invalid or unknown and has not delivered the message to the receiver.

Information from external system (if available)

Mail server host, port from SCOT

553 5.7.1 <sender address>: Sender address rejected: not logged in 

Basis release independent

XS855, Sender address rejected, not logged in, 553 5.7.1, No delivery as sender or receiver address is invalid, 855, 553, 5.7.1 , KBA , BC-SRV-COM , Communication Services: Mail, Fax, SMS, Telephony , Problem