I have a linux box which works as an smtp server. Past few months I am facing a problem, all the mails which my local lan users send to the outside world, all such mail gets bounced to root's mailbox. Also I have found that the mails which get bounced to the root's mailbox are mostly undelivered mails due to some reason.
I don't know why it doesn't go directly to the sender.
The kind of mails the root's mailbox receive is huge. Off which some are genuine.
Please let me know what are the configuration changes I need to do to resolve this problem.
This part of your problem is not a problem - it's suppose to do this. If you check your aliases file, postmaster is probably set up to go to root. It has to be going to someone. You could have it go to your account.
RFC 822 - Aliases required by the mail protocol
postmaster: root
MAILER-DAEMON: postmaster
You can change it, but it has to go to a person so errors in your configurations will be known and can be fixed.
If your 'users' are sending to dead addresses in such quanitity that it's overwhelming you, you may want to talk to your 'users'. It may be they are spamming...