[Exim] System filter and $local_part

I'm trying to set up the following scenario:

A system filter check all incoming messages if they contain '@mbs.gtn.com'.
This is our 'old' domain and will be replaced by 'mbs-software.de'
completely.

Now all users getting mails for the old address should get a reminder that
someone is still using this address and they should inform the originator
to change it.

Approach is the following:

if first_delivery and
   $header_to: contains @mbs.gtn.com
then
    mail to $local_part@???
         expand file /root/exim/sometext.txt
endif

The problem is: $local_part _always_ contains '<message filter>', which is
not quite what I want, and this leads to a mail address of
'>@mbs-software.de'

message_filter_reply_transport is set to address_reply (autoreply driver)
Used Version of Exim: 3.15

Changing this to calling a script didn't change the behaviour - $local_part
remains at '<message filter>' Using a per-address filter (as specified in
the documentation) works, but the logfile still shows '>joe' instead of
only 'joe' (when using joe@mbs-software/joe@??? as examples)

Is this a bug, or just a misunderstanding of filters?
Logfiles and example-configs on request.

TIA.

-- 
Jochen Erwied <Jochen.Erwied@???>   Phone: +49-2151-7294-24
mbs GmbH, Roemerstr. 15, 47809 Krefeld          FAX:   +49-2151-7294-50