Configuring Active Directory to use External LDAP Server for Authentication

Hi,      Can anybody help in this matter.I want to integrate Active Directory with External LDAP Directory. Means if a user login then user request goes to AD Server if not found it searches from external LDAP Server which might any LDAP Server and gives the user login.     Now, send attachments up to 25MB with Yahoo! India Mail. Learn how. http://in.overview.mail.yahoo.com/photos