Exim4: incoming Internet mail to internal mail server

[johnb@punt.nl]

I'm trying to configure Exim4 on Debian 6.04 for incoming mail only. Exim should accept all mail for domain "mydomain.example" from Internet and forward to another mail server (internal.existingdomain.example). How can I configure Exim4 to do that?

dc_eximconfig_configtype='satellite'
dc_other_hostnames='mydomain.example'
dc_local_interfaces=''
dc_readhost='mail.mydomain.example'
dc_relay_domains='mydomain.example'
dc_minimaldns='true'
dc_relay_nets=''
dc_smarthost='internal.existingdomain.example'
CFILEMODE='644'
dc_use_split_config='false'
dc_hide_mailname='true'
dc_mailname_in_oh='true'
dc_localdelivery='maildir_home'

Bottom part of a SMTP session:
rcpt to: <test@mydomain.example>
550 Unrouteable address

mydomain.example is not yet on public DNS, internal.existingdomain.example is on public DNS.

Thanks!

John