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.0.0 Other undefined Status
Other undefined status is the only undefined error code. It
should be used for all errors for which only the class of the
error is known.
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.
ftp://ftp.rfc-editor.org/in-notes/rfc1893.txt