In a multiserver setup, the client server, will only have a copy of it's data or the whole database? For example, let's assule 4 mailserver pointing to a single ISPConfig installation. Each mailserver has some mailboxes. db on mailserver1 will have only it's records, so any filter for server_id in the query is not needed, right?
That depends on the server settings that you had choosen for this slave server. If you selected that this slaveis a mirror, then the server ahs the records of the master server. This setup is used for scaling large mail systems or high availability. If the server is not a mirror, then it has only its own data.
It's not a mirror but a mail server configured as client from ISPConfig installation. So, any server_id filter in queries should not be needed.
I've made a test installation with one interface server and one server-only client. Is /interface/ directory needed on the client? Actually ISPConfig is copying that directory on all server, even if web interface is not needed.
The inteface directory is not required on slave servers that are not used as mirrors for the interface.
