This error means that there is an issue with the mail server that has been configured for use with the Send Message (SMTP) step and will need to be investigated by the customer's I.T. provider.
The issue may be that the send connector has not been configured to relay to external domains.
Relay in email is now untrusted way of sending emails. I basically just forwards the email as someone without actually going through a mailbox (sent). It is a very good way of sending spam messages. Hence why it is becoming less excepted and blocked.