5.X.X Permanent Failure A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery. X.5.X Mail Delivery Protocol Status The mail delivery protocol status codes report failures involving the message delivery protocol. These failures include the full range of problems resulting from implementation errors or an unreliable connection. Mail delivery protocol issues may be controlled by many parties including the originating system, destination system, or intermediate system administrators. X.5.0 Other or undefined protocol status Something was wrong with the protocol necessary to deliver the message to the next hop and the problem cannot be well expressed with any of the other provided detail codes.
X.5.X Mail Delivery Protocol Status The mail delivery protocol status codes report failures involving the message delivery protocol. These failures include the full range of problems resulting from implementation errors or an unreliable connection. Mail delivery protocol issues may be controlled by many parties including the originating system, destination system, or intermediate system administrators. X.7.X Security or Policy Status The security or policy status codes report failures involving policies such as per-recipient or per-host filtering and cryptographic operations. Security and policy status issues are assumed to be under the control of either or both the sender and recipient. Both the sender and recipient must permit the exchange of messages and arrange the exchange of necessary keys and certificates for cryptographic operations. X.7.0 Other or undefined security status Something related to security caused the message to be returned, and the problem cannot be well expressed with any of the other provided detail codes. This status code may also be used when the condition cannot be further described because of security policies in force.