Manitou-Mail stores everything into the database, including user preferences and configuration entries.
It is recommended to periodically backup the entire database by running the default PostgreSQL backup program, pg_dump, on the manitou-mail database(s). Example:
postgres$ pg_dump -Fc -d manitou -U manitou >mail.dump
Other methods of backup are possible, such as incremental backup with the transaction logs or master-slave replication, but they're outside the scope of this documentation. Please refer to the PostgreSQL documentation and resources for more information.