The per-recipientfield contains a transport-independent status code which indicates the delivery status of the message to that recipient. This field MUST be present for each delivery attempt which is described by a DSN.
Status codes thus consist of three numerical fields separated by dot ("."). The first sub-field indicates whether the delivery attempt was successful (2 = success, 4 = persistent temporary failure, 5 = permanent failure). The second sub-field indicates the probable source of any delivery anomalies, and the third sub-field denotes a precise error condition, if known.
In this example, DSN field is:
The return value is marked bold.