[Pkg-exim4-users] Route only outgoing mails to smarthost

[fn_online at arcor.de]

Hi,

I'm having a problem with my exim configuration. I'll try to explain what's going on as good as I can.
Let's call my server "exim.mydomain.com". There's also the domain's standard mailserver ("mail.mydomain.com").
Both servers are supposed to handle mails to *@mydomain.com in a certain way. 
The "mail.mydomain.com" only handles mails to certain addresses (e.g. a at mydomain.com) and routes everything else to my "exim.mydomain.com".
My "exim.mydomain.com" only handles mails to some other addresses (e.g. b at mydomain.com), every other address is supposed be rejected (e.g. c at mydomain.com).
All outgoing mail is supposed to be routed to the mailserver (smarthost).

My config:

dc_eximconfig_configtype='satellite'
dc_other_hostnames='mydomain.com'
dc_local_interfaces=''
dc_readhost='mydomain.com '
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='mail.mydomain.com'
CFILEMODE='644'
dc_use_split_config='true'
dc_hide_mailname='true'
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'

It's working quite well. I get the mails going to b at mydomain.com and the mails to c at mydomain.com get an undeliverable message (#550 Unrouteable address).
The problem appears when I try to send mails. Mails to other domains work fine. But mails to a at mydomain.com can't be routed.

>exim4 -bt a at mydomain.com
R: system_aliases for a at mydomain.com 
a@ mydomain.com is undeliverable:
  Unrouteable address

I've tried a few things to solve this, like setting mydomain.com as a relay domain or changing the smarthost router to allow local_domain. With these changes the mails to a at mydomain.com get routed, but in both cases the incoming mails to c at mydomain.com will also be routed back to the smarthost (causing a loop between my exim server and the smarthost, because none of them wants to handle this address).

Is there some way to configure exim to route the outgoing mails to the smarthost, but still reject (undeliverable) the incoming mails to c at mydomain.com?

Thanks
Frank