A major part of how the current exim.conf files (both the one supplied by DirectAdmin here, and the one I write and maintain here is to not even consider accepting incoming mail unless the sender is an authorized relay, or the destination address is hosted on the server. The method used to determine if the destination address is hosted on the server is intimately tied to how/where DirectAdmin defines virtual and username email addresses.
So the first step will be to rewrite that section of the exim.conf file.
Then the next step will be to route all email to the final destination server.
I haven't really studied the changes which need to be made because by default we do NOT scan outgoing email to see if it's spam.
While both are doable, neither are trivial.
We (NoBaloney Internet Services [nobaloney.net]) can certainly write custom exim.conf files under contract, but there would be an hourly rate charge.