For the longest time we have been using Exchange Transport rules to block a particular group of users from sending and receiving emails to and from the Internet. The rule would read something like, if sent from a member of this DL and destined for the outside, send an NDR back to the user unless it is to a particular address in another DL, then let it go. (exceptions). We did the same thing for receiving inbound Internet email using the same logic.
This has worked for years and recently we have been purchased by another company and they want to be able to send and receive emails to and from these users. The problem I am having is the rule only allows for individual email addresses to be added as exceptions, not entire domains and since this company has thousands of users, I certainly can't list individual addresses as exceptions to these rules.
I can't add the domain as an accepted domain in Exchange because we do not receive email for it. For now, I had to disable the transport rules until we can think of a better solution. Does anyone have any ideas on how to best handle this? We are running Exchange 2010 SP1.