Lookup for "553 4.7.6"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (553)
Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect]
Enhanced Status Code Details (4.7.6)
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.