Can I block External e-mail while allowing unauthenticated internal e-mail in Exchange 2007?

We have some products that generate unauthenticated e-mails that need to go to internal users who are not allowed to receive e-mail from outside of our organization.  Typically the way to block some users from receiving e-mail from the outside is to only allow e-mail from authenticated users.    If we take off the authenticated users only restriction they can receive these e-mails but then they get all of the outside e-mail as well.

I already use a distribution list and a transport rule to keep these users from sending e-mail to the outside.  I tried creating Transport Rules to block members of the internal e-mail only distribution group from receiving e-mail from outside the organization so I could allow unauthenticated internal e-mail but it doesn't appear to work.  Is there a way to do what I need to do?
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fpthreeAsked:
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MesthaCommented:
One method would be to route the email from these systems via another server that does authenticate when sending. A windows 2003 server with IIS and SMTP installed would probably do the job for you quite easily.

The reason the rule isn't working is because I don't think Exchange will see unauthenticated email as being internal. It has no way of knowing whether that message has come from a server located on or off site, all it knows is that it wasn't authenticated.

Simon.
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