Redundant Postfix Servers

Please explain your use of the postfix servers, are they serving any local mail or are their use is solely for relaying outgoing mail i.e. taking the burden from the exchange dealing with outgoing queue processing.

Load balancing sort of having a Loadbalancing switch infront of the server, you would use DNS entry for smarthost.internal.private which will internally resolve to as many servers as you setup with a TTL 30, 25,60,90 seconds.

In outgoing situation, you can not balance/distribute the queue.  It is locally stored and locally processed.  if a system goes down, the only way to get the messages out is to fix and boot the server, or gain access to the data and .... reprocess it.
Usually, if a message did not make it on the initial connection, it is likely not going to make it on the nth i.e. the recipient domain/user is more likely than not to be invalid.

a failed server, will cause your exchange to requeue the outgoing messages until it hits a functional one.  Adding scripts that check on the availability of the mail server and those that are not available removed via DNS updates...

Commonly, mail servers are setup in tandem on the incoming side.
Multiple head units with one common storage where the mailboxes are stored.

In case if there are mails in the Postfix Queue & due to some reason the server fails & by which it is not able to send e-mails.
Once the server is restarted will the mails in the Queue will be available & will it automatically be sent .
Will there be any loss of e-mails

Messages in the queue will be retransmitted when the system comes up, but if it is down for long enough, one attempt might be still made before the sending of the message is given up on because the queue lifetime has been exceeded.

Are the mails in the postfix queue only outgoing i.e. replies as well as bounces of undeliverable mail.

A possible loss of email always exists when the mail server  dies. Depending on the reason, the drive could be pulled the outgoing queue looked at and messages if needed copied so that they can be transmitted.

Some email loss is more significantl than others, so you need to be more specific.
I.e. you have hundreds of emails in the queue that are NDRs to the sender.  Would you consider those email loss significant?  In many cases, those emails are spam sent to your domain......

I will be using this Postfix for both outgoing & to receive the NDR.

Please explain what do you mean to receive the NDR,

Is your postfix sitting between the exchange and the internet?
I.e. it is filtering the incoming and passing them along to the exchange at the same time deals with any rejected by exchange or responses from the inside to deliver them out?

In that scenario, if the server dies as soon as an accepted message comes in, it may not have the opportunity to pass it on to the exchange in which case it will introduce a delay or possible message loss.

Yes my posftix will be sitting between the exchange & the internet.
Most of my mail flow will be only outgoing. But I want my NDR mails also to be received.
I need those NDRs for analysis purpose.

NDRs are sent to the sender of a message, it will not be delivered to "you" until they double bounce <#@> i.e. the recipient is not valid or could not be reached and the sender is not valid or can not be reached.
At that point if configured, the double bounce could be delievered to the local postmaster account or an account configured to receive them.

You can use maillog/exchange smtp data to analyze some data.  Guess it depends on what data you need.

I am the sender & I would like to receive the NDR

Thanks