Lookup for "221 4.4.4"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (221)
Enhanced Status Code Details (4.4.4)
Unable to route
The mail system was unable to determine the next hop for the message because the necessary routing information was unavailable from the directory server. This is useful for both permanent and persistent transient errors. A DNS lookup returning only an SOA (Start of Administration) record for a domain name is one example of the unable to route error.