Lookup for "251 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 (251)
User not local; will forward to
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.