o "Error: authentication-failed" - the receiver could not authenticate the sender. o "Error: decompression-failed" - the receiver could not decompress the message contents. o "Error: decryption-failed" - the receiver could not decrypt the message contents. o "Error: duplicate-filename" - the message payload contained a filename already received by the backend server. o "Error: illegal-filename" - the message payload contained a filename that could nor be processed by the backend server. o "Error: insufficient-message-security" - the content of the message was not appropriately enveloped according to the agreed-upon message security. o "Error: integrity-check-failed" - the receiver could not verify content integrity. o "Error: invalid-message-id" - the receiver could not parse the value of the Message-ID header because it was not syntactically correct. o "Error: unexpected-processing-error" - a catch-all for any additional processing errors. o "Error: unknown-trading-relationship" - the receiver could not correlate the AS2-To/AS2-From header values to values known to the system.