Stefano point out that we don't need this class anymore after i did the work ;-P
We can simply use the javamailstore which joachim develop and use mstor as javamailstore provider.
Here is what you have todo to get it work:
1. Get the javamailstore-mailrepository-5.jar from http://issues.apache.org/jira/browse/JAMES-461 and place it to /path/to/james/apps/james/SAR-INF/lib/
2. Change the mbox config to:
<repository destinationURL="mstor://var/mail/" postfix="/" type="MAIL"/>
Thats all. I whould be +1 to include joachims work finally to james trunk