I've made my own auditor. When I raise Reject Exception it is caught by web interface and I get an alert 
"Edit Error: ..." - it is OK.

But when I try to do the same via mail interface Roundup doesn't catch my Reject exception and I get a message:

"An unexpected error occurred during the processing
of your message. The tracker administrator is being
notified."

However, When I raise ValueError exception instead of Reject exception - everything is OK. It is cougth either by web or mail interface.

-tt.