Email status messages - what do they mean

With one of my clients I need to monitor email bounces from our mailing lists. Normally this goes relatively smoothly, as either the email goes through, or it bounces back with a 550 Recipient not found message. Sometimes the messages I get back are less familiar to me. Here is the appendix from RFC1893. Hopefully this will be a useful guide:

2.X.X   Success
Success specifies that the DSN is reporting a positive delivery action.  Detail sub-codes may provide notification of transformations required for delivery.
4.X.X   Persistent Transient Failure
A persistent transient failure is one in which the message as sent is valid, but some temporary event prevents the successful sending of the message.  Sending in the future may be successful.
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.1.0   Other address status
X.1.1   Bad destination mailbox address
X.1.2   Bad destination system address
X.1.3   Bad destination mailbox address syntax
X.1.4   Destination mailbox address ambiguous
X.1.5   Destination mailbox address valid
X.1.6   Mailbox has moved
X.1.7   Bad sender’s mailbox address syntax
X.1.8   Bad sender’s system address

X.2.0   Other or undefined mailbox status
X.2.1   Mailbox disabled, not accepting messages
X.2.2   Mailbox full
X.2.3   Message length exceeds administrative limit.
X.2.4   Mailing list expansion problem

X.3.0   Other or undefined mail system status
X.3.1   Mail system full
X.3.2   System not accepting network messages
X.3.3   System not capable of selected features
X.3.4   Message too big for system

X.4.0   Other or undefined network or routing status
X.4.1   No answer from host
X.4.2   Bad connection
X.4.3   Routing server failure
X.4.4   Unable to route
X.4.5   Network congestion
X.4.6   Routing loop detected
X.4.7   Delivery time expired

X.5.0   Other or undefined protocol status
X.5.1   Invalid command
X.5.2   Syntax error
X.5.3   Too many recipients
X.5.4   Invalid command arguments
X.5.5   Wrong protocol version

X.6.0   Other or undefined media error
X.6.1   Media not supported
X.6.2   Conversion required and prohibited
X.6.3   Conversion required but not supported
X.6.4   Conversion with loss performed
X.6.5   Conversion failed
X.7.0   Other or undefined security status
X.7.1   Delivery not authorized, message refused
X.7.2   Mailing list expansion prohibited
X.7.3   Security conversion required but not possible
X.7.4   Security features not supported
X.7.5   Cryptographic failure
X.7.6   Cryptographic algorithm not supported
X.7.7   Message integrity failure