[Exim] Problem with copied mails ...

Hello,

i "installed" a filter/router for my outgoing mails in order to keep all
outgoing mails in one mailbox.
The transport is handled by a perlscript named copy_outgoing (as you can
see).

## begin part of exim-config
remote_smtp_filter:
driver = smtp
transport_filter = /sbin/copy_outgoing

filter_remote:
driver = lookuphost
transport = remote_smtp_filter
## end part of exim-config

The filter works fine so far and the script looks like this:

## begin perlscript

#!/usr/bin/perl
open(FILE,">>/var/spool/mail/mailout");
while (<STDIN>) {
print FILE $_; # print to file for saving
print $_; # print to stdout for delivery
}
close(FILE);

## end perscript

The problem is, that the created outputfile is not recognized as a mailbox
by grepmail etc.
Maybe someone could give me a hint what's wrong with my script/filter etc.

Thanks in advance, Roland