Just following myself up -- I tried doing a test like:
exim -f jra@??? -d -bt test@???
and from what I can tell, that worked just fine with the final result being delivery to the mailman_transport and no errors.
So, the oddity is that the exim test passes for a remote sender, yet an actual message from a remote sender bombs with "550 - unknown user."
Thanks for any help.
John