As far as I know, you can define which tables shall be replicated. But Falko might help you more with this topic as I'em not that familar with mysql replication.
Ok I think I found the way how to replicate just some tables from a database. (I've just replace "replicate-do-db" to "replicate-do-table" and repeate this command for all the tables that I want to replicate). The question now is: which tables are releated with the email part of ISPConfig? Thanks Michele