[exim] how can exim 4.54 check for valid 'non-local' users w…

I am asking this question, in the hope I can come up with sensible
suggestions to my ISP who runs on debian exim 4.54 and is the MX for
about 15 os so of our domains and then routes our email to worldwide
remote uucp sites.

In our current setup our 'uucp email ISP' does not know our remote
uucp 'local user lists' (or any list of valid email addresses at our
remote uucp sites). So as our MX that routes into our uucp queues they
have to currently accept every conceivable 'local' email address at
our remote uucp site.

This is of course has the consequence that we now get thousands of
very imaginative emails to our uucp sites for users that do not exist
locally.

Can any of you suggest anything at all, that we as the remote uucp
sites could be uploading to the 'uucp email ISP' s MX machines that
would allow the exim routing process running there to actually check
which emails to accept, and which to drop or return, right there and
then, whilst the smtp process is going on?

We do have bandwith and internet availability/reachability
restrictions for a number of those uucp remote email sites, so looking
up a service like ldap for valid user names remotely or any such
remote access doesn't seem an option.

What we could do quite easily, is compile a comprehensive catalogue or
list of all valid and accepted 'local' (local to the remote uucp
sites) email addresses, and have those uploaded to the ISP's MX
machines in a timely and automated fashion. (say, everytime we would
add a local user or add an email alias or list or such thing, we would
trigger a re-build and re-upload of the full list)

Any ideas or hints received gratefully.

--
Urs Rau