[exim] relay and final rejection for exchange org.

I use exim to filter mail for an exchange organization.

I'm about to start a migration and need to share the SMTP domain.

According to the MS docs, I need a final destination that will reject
undeliverable mail and thus prevent a mail loop.

I want Exim to accept mail destined for the shared domain from the
internet but reject or pipe to null if the same mail is relayed back
from the internal exchange servers.

I'm trying to avoid building another box. Can I bind a different exim
config to a separate network interface?

I have the R4 book. Any pointers would be appreciated.

Thanks

Scott