Active directory authentication with postfix

You don't really need active directory to verify if an email address exists.
postfix can do this with SMTP.
see
http://www.postfix.org/ADDRESS_VERIFICATION_README.html

/etc/postfix/main.cf:
    smtpd_recipient_restrictions =
        permit_mynetworks
        reject_unauth_destination
        ...
        reject_unknown_recipient_domain
        reject_unverified_recipient

You can also have postfix cache the lookup results if you want to.

    address_verify_map = btree:/var/lib/postfix/verify

If you wanted to use Active Directory for SMTP AUTH for your remote users,
see
http://www.linuxmail.info/active-directory-cyrus-sasl-authentication/

We need  postfix do LDAP lookups  LDAP and query for valid recipients then relaying to the exchange server

Why does it have to use LDAP?

http://www-personal.umich.edu/~malth/gaptuning/postfix/

We dont need to pull users' SMTP addresses from your Active Directory because it will take some processing time and not practical.We need to have a direct look to AD this is a valid mailid or not.

AD Authentican is for avoiding bulk mails

You don't need to use AD to know if the email address is valid.
There are other much easier ways to achive this.

Postfix has a special feature desgined for exactly this purpose.
It is call email address verification and can be use to verify the validy of sender address or recipient address.

See my first comment for details.

We need only our internal mail authentication via AD.

Not recvd any practical solutions