Current setup is Exchange 2003 SP2. We have distribution groups used primarily for internal communication across multiple business units which are not all Exchange users or members of our domain. Generally we want to block all external message from these groups with limited exceptions. We want specific external SMTP senders to be able to send to these groups, these are users from our other business units. Is there any way to do this with the built in features of Exchange 2003 SP2?

Exchange general tab of the group, you can set it to allow from authenticated users only, from everyone, from everyone except (then list users, groups contacts) or only from (list users, groups contacts). Any external SMTP addy's would have to be added as AD contacts.

Thanks for the reply. While it didn't solve the issue it did help me look at it in another way. If you set the only from option and add the distribution group as the only allowed sender which in my case includes the other business units as recipients I'm able to get the desired result.