Is there any way of 'bouncing' email that is sent to users that don't exist?
It sounds to me like you're asking for a non-delivery report (NDR) to be generated and sent to the originator notifying them that the email address they sent to does not exist at your organization. If that's the case, then you'll want to make sure this is configured on your Default Global settings in Exchange. I'm not sure which version of Microsoft Exchange Server you're running, so I'll assume either Exchange 2000 or Exchange 2003.
You need to launch Exchange System Manager (ESM) and expand the Global Settings container. Click on the Internet Message Format node and then select the Default * domain in the right-hand panel. Double-click and switch to the Advanced tab. You'll want to make sure that "Allow non-delivery reports" is selected. This will enable all of your external users to receive an NDR whenever they send to addresses that don't exist within your organization.
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