Lookup for "550 4.4.5"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
Enhanced Status Code Details (4.4.5)
Mail system congestion
The mail system was unable to deliver the message because the mail system was congested. This is useful only as a persistent transient error.