[exim] Transport to route local mail to bSMTP

I am building a community service to transfer mail from offline locations
via physical carrying of bSMTP data.

I have managed to setup the router and transport to write bSMTP on the
offline exim4. This is then collected, transferred to a connected server
and then sent using exim4 -bS , but I am struggling to take received mail
via smtp and writing to bSMTP for transfer in the other direction.

We need SMTP received over TCP written as bSMTP into folders based on the
subdomain of subdomain.domain.com. There is no need for mail to be
delivered locally.

Any pointers gratefully received!