[exim] Unroutable address when bouncing a message to a virtu…

Hi,

I'm having the problem that I get "Unroutable address" for the sender if
a message bounces which was sent by a virtual user.
These users can receive messages comming from outside (SMTP) by the
following router:

maildrop_virtual:
debug_print = "R: maildrop_virtual for $local_part@$domain"
driver = accept
domains = +local_domains
# acl_c0 is set in the RCPT ACL
local_parts = ${extract{uid}{$acl_c0}{$value}}
transport = maildrop_pipe
require_files = /etc/maildroprc:+/usr/bin/maildrop
no_verify
no_expn
user = Debian-exim
group = vmail

acl_c0 contains various key/value pairs like uid and mailbox for calling
maildrop in the transporters. It's populated by a ldap lookup (in
acl_smtp_rcpt) and I'm using that to test if the message is for one of
the virtual users.
After adding another ldap lookup in acl_not_smtp it also works with
message comming from -bm.
But in case of a non-interactive incomming message (-bt or bounce) it
doesn't work. Such messages get routed without the local_parts-check in
above router, but removing that can't be the solution since it would
accept all messages at this point.

So what can I do to check for virtual user as recipient for a bounced
message?

Regards,
Juergen