Clustering is a way to share the load of your services across a network of computers. For example, one server for smtp, another for pop3 and yet another for imap. Unless you have a very large volume of traffic such as a university or isp, it would not be very practical. It is a little too complicated an issue to do it justice in a forum environment but you can google for "sendmail cluster" to find some papers on the subject. We can help you with any specific questions you may have though.