Recieve Connector

One of our programmers setup an application on 100 computers (same subnet) that relays email off our exchange 2007 server to internal and external recipients. Rather then setup an receive connector that allows anonymous access to entire subnet, is there a way to setup a new recieve connector that will only allow 3-6 domain users (the program uses the same email accounts to send the emails) to connect on port 25 and send mail from any of those 100 computers to internal and external recipients. Originally, I setup an anonymous relay connector and explicitly defined the ips but there are simply too my ip's to list at this point. I also don't want to allow the entire subnet to relay mail off our exchange server. Any help would be greatly appreciated.
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Chris DentPowerShell DeveloperCommented:

Can the application authenticate for SMTP? If not, you're a bit stuck with the relay.

Of course, if the mail only needs to be delivered to other internal users you don't need a relay at all, but I imagine that would be far too easy.

FREDARCEAuthor Commented:
I'm going to look into secure smtp for those users. They do need to send internal and external.
 Thanks Chris.
Setup a hub transport rule to allow/disallow mail to those users.

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