I want to make backup Exchange 2003 Serverto run paralell with primary Exchange server,same likePrimary domain controller and backup domain controller, so that if primaryExchange goes down the backup exchange server become active and work like same as primary Exchange mail server.

Hi Abdul, Well, in Exchange there isnt any option to set like primary and secondary domain controller. You can create an active/passive two node cluster which works like same, one will be active and one passive and when active goes down passive will take incharge, but you need to build a new cluster and existing exchange server can not be used while building. Deploying Exchange Server 2003 in a Cluster http://technet.microsoft.com/en-us/library/bb123612(EXCHG.65).aspx