Lookup for "211 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 (211)
System status, or system help reply
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.