Lookup for "211 5.7.6"
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 (5.7.6)
Cryptographic algorithm not supported
A transport system otherwise authorized to validate or decrypt a message was unable to do so because the necessary algorithm was not supported.