To customize a delivery failure report
Specify a custom DFR message for each rule. This will let the user know why their message was rejected.
Specify a enhance MAIL codes DFR message ala RFC 1893.
Maybe even send the rule as part of the DFR message.
The support of generic DFR messages will be a problem for some set up. If we were able to return a custom DFR message based on the rule that was applied to the DFR, it will be easier.