Relay SMTP Authentication to remote mail server through IIS SMTP Virtual Server

You can set the IIS SMTP Virtual Server to ask for authentication as a restriction for relaying, and you can set the method of authentication through "Access Control".
But the fact is that it can only authenticate on the local users/passwords database of the RELAYERSERVER itself(or the shared Active Directory Database), and there is no way to authenticate to a remote machine(MAILSERVER) without having any shared database between relayer and the mail server (no Exchange and Active Directory schema).
Is there any add-in/Extension to the IIS SMTP Service so that the authentication can also be relayed to a "remote MAILSERVER"? Something like relaying AUTH command of SMTP protocol to the remote mail server.
This functionality is so vital for me!
So?

Can you not allow access of the IP of the remote mail server instead?

Tnx degsy, but sorry maybe I haven't described it well enough.
You see I want any user who has a mailbox from my internal domains to be able to use my relayer, but forced to authenticate first through the relayer. It doesn't matter where the user is located.
Do I make it clear this time?

All I can suggest is a good site for mail-related stuff is www.slipstick.com although it's mostly Exchange Server/Outlook. You can look around and see if there is any IIS SMTP help there.

Maybe LDAP routing? I don't know much about configuring IIS SMTP, I don't use it myself.

Don't you just hate people that respond to forums when they do not know about the subject - Clearly the subject states IIS SMTP Virtual Server

You cant come along and say "I dont know much about configuring IIS SMTP, I dont use it myself."

How annoying - Thats why i have replied to a two year old post.. !