[Exim] check localparts on backup mx

Hi.

I have a list of all valid users of a domain I'm running a backup mail
server for. Currenly that domain is simply listed in relay_domains and all
local parts are being accepted.

Since I have a list of all valid users I would like to reject all RCPT
commands to users of that domain that don't exist.

I'm stuck. I don't know whether I should be using a director or a router. I
first thought of a dummy director that should have domains and local_parts
options set, but the domain is not local, so directors would not be tested.

Thank you.

--
Kirill