Exchange 2010 SP2 Rollup 4.
I have a unique situation where I need to restrict users from sending to anyone that is not a member of a local distribution group (all in the same email domain). I want a closed cell of 20 users that can only send and receive from each other. I have already used delivery restrictions to prevent them from receiving email from anyone else, but now I need to lock down sending. They must not be able to send to any address outside a specific group, OU, or email domain. If it helps, I have them broken out into their own GAL that is segregated from the rest of the Exchange org as well.
I had thought that 2010's transport rules could facilitate that, but all of the options I'm seeing would require a negative operator (NOT, NE, !=) and that doesn't appear to be an option. I hope I'm missing something. It wouldn't be the first time.
Is there a way to do this? If so, what would transport overhead look like? I'm pretty comfortable with EMS so Powershell it up if required.
Essentially, I am looking for an "approved recipients" restriction for a group of users.