>> As to the ERROR-IS codes, they don't have to be complicated. When an >> error condition is detected, they would be an OPTIONAL way to further >> describe the error. The only requirement would be that no fales >> descriptions be sent. The coding can be structured with varying amounts >> of information. Whatever follows the number can be text taken as a >> comment. > >What will programs do with this information about the cause of the >error? It seems to me that the only reason for putting such >information into a header is that a program might process it in some >way. A person can see what the error was by reading the text of the >rejection notice. Well, if there were some clear indication (a separate digit, or dedicated values in a digit) that indicated a condition that is likely to be temporary (systems disk space full, gateway queue full, gateway unavailable) versus a condition for which no relief was in sight (addressee unknown, restricted gateway), then a suitable program might schedule a retry for the temporary cases.