It seems that you can connect to the server with Address, but you need Host for authentication on the server.
Is that right?
No, if the XMPP service is configured properly, i.e. with DNS SRV records, then you only need to the XMPP domain set as service name. Smack will use the information from the DNS SRV records to decide which host/port to connect to. This will also work if the XMPP domain's A RR resolves to the host that serves as XMPP server for the domain. Only if this is not the case, you have to manually specify the host and port.