[exim] Backup-MX user lists

Hi,

I'm hosting email for a handful of 'virtual' domains.

There is more than one MX for the domains:- the "final destination"
which sits on my adsl line, and another "backup-MX" under my control
permanently hosted elsewhere.

The backup-MX currently uses recipient callout's to validate the local
part before accepting the mail. It's this callout I'd like to
avoid... What I would like is for the backup-MX to reject mail with
invalid username *without* consulting the final destination.

Does anyone have a canned example I could use as a starting point
please? because I'm struggling... something using a flat file
structure would be ideal.

On the final destination host (my home box) I use the following config
(cribbed from various howto's):

vdom_aliases:
      driver = redirect
      allow_defer
      allow_fail
      domains = dsearch;/etc/exim4/virtual
      data = ${expand:${lookup{$local_part}lsearch*@{/etc/exim4/virtual/$domain}}}
      retry_use_local_part
      pipe_transport   = address_pipe
      file_transport   = address_file
      no_more

The format of /etc/exim4/virtual/$domain is similar to the /etc/aliases file.

Thanks for any help!