Hello Saleena,
What is the error on the mail_logs when an auth fails?
the SMTP auth query on LDAP make sure it's set, and also set in the auth profile + listener to use.
One thing to note is LDAP usage with auth is the LDAP bind - it looks for the attribute you are configuring and binds against the password for the user in AD.
Let us know the error, perhaps some ESA gurus on the forums can assist as well.
Thanks,
Mathew