[Exim] Forwarding unknown users to another host

Greetings everyone,

I'm a new list subscriber, but a long-time Exim user. I am trying to set
something up that I thought would be simple, after having read the FAQs,
but it's not working out.

I have a FreeBSD machine acting as an internet gateway/firewall and mail
server. Behind it, on the internal network, is a Windows 2000 server
running Exchange 2000. I want the FreeBSD box to handle mail for all of
its local accounts and virtually-hosted domains, but to forward mail for
any address that it doesn't recognize to the Exchange server. From FAQ
Q0403, I added the following router to the end of the Routers section in
the configure file:

  unknown:
     driver = manualroute
     transport = remote_smtp
     route_list = * exchange.domainname.org
     no_verify

where domainname.org is the name of the domain (I censored it out for
security reasons). The hostname exchange.domainname.org resolves to the
private IP address of the Exchange server.

When I attempt to send mail from outside to my e-mail address,
bnewall@???, it bounces back with the following:

  bnewall@???
    SMTP error from remote mailer after RCPT TO:<bnewall@???>:
    host mail.domainname.org [ww.xx.yy.zz]: 550 Unrouteable address

Where mail.domainname.org is the hostname of the FreeBSD box, and
ww.xx.yy.zz is its IP address.

I'm stumped, and I'm not sure if it's an Exim problem or an Exchange
problem.

Any ideas?

Thanks in advance!

*********************************************************
*    Bryce Newall    *    Email: data@???    *
*               www.dreamhaven.org/~data                *
*  "Computers make very fast, very accurate mistakes."  *
*********************************************************