It's really nice standalone mail server.
Written in JAVA, can be scaled to high numbers of nodes.
I have used mysql based setup

| $JAMES_HOME/conf/database.properties

database.driverClassName=com.mysql.jdbc.Driver
database.url=jdbc:mysql://localhost/james
database.username=james
database.password=james
vendorAdapter.database=MYSQL
openjpa.streaming=false

Now download mysql jdbc driver from dev.mysql.com