Today I managed to finish integrating the HBase mailbox implemenation for James. I committed all project code based that I developed during Google Summer of code, including the integration tests.
I also added a spring context definition so now it's very easy to use. Just fire up James and supply it (put on the class path) with the xml configuration file for your HBase cluster. Users and domains can also be stored in HBase or in another supported repository (like a database).
This flexibility is achieved because the way James is built. All the important parts are spring components that can be interchanged easily. Also, if you did not already know: James is moving towards OSGI.
So what are you waiting for, go try out the new James goodies on your cluster and send us some feed-back if you have time.
Just remember you have to get the latest trunk version for this and do some compilation. Here you can get detailed instructions on how to do that.
If you are not in a hurry you can wait until we make a new release or check the nightly builds.