By defining priorities in the MX record.
Load balancing is done between 2 or more servers where the load is distributed between servers. whereas, fault tolerance is done between 2 servers where one server acts as a primary and the second server acts as a back up server for the primary server. In case of failure of primary server, the second server(back up server) becomes the primary server.
A Cluster Server
No!
The primary uses of a mail server are to send and receive messages for its users.
The prefered DNS server for the site is the server which sends the dns zone updates to all the other servers in the site. The primary DNS zone is created on the server and it has the authority to send changes in zone to other servers
The best way to maximize the space on the application server by clustering the servers is grid computing. You can read more about it at http://www.oracle.com/technologies/microsoft/docs/grid-computing-windows.pdf Clustering allows you to run multiple services on one machine or on multiple machines. Servers can be arranged to maximize space with multiple servers running.
virtual hosting
It is technique to reduce the server load and www4 simply means there are multiple servers.
Notify List
It is called virtual hosts, and it is supported by Apache.
Dedicated server
You simply change the port number in the minecraft server properties