The available modules are the Mailbox API, Mailbox Store(the base and utility classes) and the different implementations we propose (Memory, Maildir, JPA, JCR and HBase).
A module for tooling is also available.
You can read a tutorial on how to build Apache James Server on http://james.apache.org/server/3/dev-build.html.
The GIT URLs and the project names must be adapted, but the requirements and process are the same and you can inspire from the build the Apache James Mailbox libraries.