[exim] Pipe-Transport / Permission Issue

Hello,

i have this procmail-transport:

procmail:
         driver=pipe
    command="/usr/bin/procmail -d ${local_part}"
    user=$local_part
    group=mail
    envelope_to_add

for some reason, the mbox files that
procmail creates are not owned by $local_part:mail
but root:mail so it seems, that exim does not start
procmail as $user:$group although exim's debug output
tells about changing uid/gid.

<....>
25922 seeking password data for user "nrittner": using cached result
25922 getpwnam() succeeded uid=1000 gid=1000
25922 search_tidyup called
25932 changed uid/gid: local delivery to nrittner
<nrittner@???> transport=procmail
25932 uid=1000 gid=8 pid=25932
25932 auxiliary group list: <none>
25932 home=/home/nrittner current=/home/nrittner
25932 set_process_info: 25932 delivering 1JPelf-0006jp-OQ to nrittner
using procmail
25932 procmail transport entered
25932 direct command:
25932 argv[0] = /usr/bin/procmail
25932 argv[1] = -d
25932 argv[2] = ${local_part}
25932 direct command after expansion:
25932 argv[0] = /usr/bin/procmail
25932 argv[1] = -d
25932 argv[2] = nrittner
25934 set_process_info: 25934 reading output from |/usr/bin/procmail -d
${local_part}
25932 Writing message to pipe
25932 writing data block fd=9 size=98 timeout=3600
25932 writing data block fd=9 size=6273 timeout=3600
25932 writing data block fd=9 size=1 timeout=3600
25910 procmail transport yielded 0
25907 journalling nrittner@???
25910 search_tidyup called
25907 procmail transport returned OK for nrittner@???
</....>

as a result the created mbox files are not readable for user $local_part.

What can do the trick ?

Thanks for your help/hints

Nico