[exim] Callouts not working

I have a server receveiving mail from the internet and it passes most of
the mail to another internal server. I wish to configure the main server
to do callouts against the second server. Both servers are running Exim
version 4.63 (Debian testing).

At present I have enabled ident calls on both servers. I have also
configured an acl_check_rcpt acl block Debian's
30_exim4-config_check_rcpt.

The domainlist relay_to_domains is set to domain1:domain2

  accept
        domains = +relay_to_domains
        endpass
        verify = recipient/callout=defer_ok

However the callouts aren't working. I assume this is because the main
server can't resolve the target host for the internal mail at the point
of this acl block.

The router to pass the mail on to the internal server is

    LOCAL_EXAMPLE_REDIRECT:
      driver = accept
      domains = ! +local_domains : +relay_to_domains
      transport = LOCAL_EXAMPLE_FORWARD
      no_more

Do I require internal MX records for domain1 and domain2 for callouts to
work?

Thanks for any help.
Rory

--
Rory Campbell-Lange
<rory@???>
<www.campbell-lange.net>