The complete set of status codes used to report mail system conditions is outlined in RFC 3463. Common class sub-codes include:
Success specifies that the DSN is reporting a positive delivery action. Detail sub-codes may provide notification of transformations required for delivery.
4.XXX.XXX (Persistent Transient Failure)
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
5.XXX.XXX (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.
For a detailed description of SMTP status codes, see RFC 3463.