We are running Postfix 2.1/Courier-IMAP /OpenLDAP / SquirrelMail based E-mail Setup.All these components run from the same box.Users mail boxes are also stored locally.
We have another same setup which is generally idle but used just for mail backup.We use Rsync for that purpose.If one server gives any trouble we just swap the IP addresses and it works.
I know it is not the best setup.I want to know what are the various options i have??I want to use these two servers effectively.At the same time i want to ensure easy switch over in case any box gives trouble.
We have around 2000 mail boxes and work under VPN.

You mean like http://www.linux-ha.org/ ?

I dont want any clustering.We dont have any shared storage.Both Postfix boxes have their own storage.We have two stand alone Postfix and we are using Rsysc at night to make them identical(storage wise). I want to have suggestion how to configure POSTFIX in such way that it perform 'store and forward'?Is it possible?Or any thing like that ?There must be some better approach.
I want both these Postfix should have independent mail storage and always iddentical.

1) How do you mean "identical"? Is there any tolerance for difference? I.e. can they sync every 5 minutes, or do they need to be constantly identical?

If every 5 minutes is OK, go with rsync. If you need them to be *always* exactly the same, then your best bet would probably be looking into storing the mail data on a set of other servers, and find some way (I don't know how, but I know it can be done) of setting up the two file servers so they "look" like one from the network and are mirrored.

2) In terms of doing an IP switch, if you have DNS running, just set one as the MX0 and the other as MX1. The clients will automatically failover to MX1 if MX0 can't be reached.