Lookup for "550 5.4.5"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
Enhanced Status Code Details (5.4.5)
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.