Wouldn't it be easier to just disable the Option to send emails outside your Company?
As usual you can modify the SMTP-Settings of your Exchange-Server and there you can define some stuff like that.
With this solution it is guaranteed that no one can send emails out of your Company with the agents.
your only option within the agent/OBIEE would be if all your users allowed to receive the reports are OBIEE users and so you can run the agent as "Recipient".
Otherwise what you look for isn't an OBIEE thing but, as PiusV say, a SMTP config.
When running the agent with a specified user all the recipients get the same content, OBIEE doesn't really use the recipient list other than just to send all the emails to the list, so you can't have a condition applied on the members of the list.
Set some security in your SMTP, at least you are sure that nobody (by mistake ... or not) send out around your numbers.