[Exim] Returning error messages using exim filter

Hi all.

I am using the system_filter.exim (windows executable content filter)
file which is currently available on the Exim website. I have found it
to make a nice job of filtering out attachments but am having problems
with the returning of the message to the original sender. There are two
reasons for this.

First, my clients all use a dial-up mail server which collects mail for
the domain via fetchmail then uses trestlemail to forward mail into the
appropriate mailboxes. As trestlemail generates messages on the command
line, they are listed as being sent by the trestlemail user as opposed
to the person who actually sent them. This means that the exim error
message is sent back to root@proxy rather than to the original sender.

Secondly, many worms these days act as though they have been sent from
someone else in the user's address book. Therefore, to send the sender a
warning message may be of limited use. The real header which is always
of use is "Return Path:" which is genuine.


I'm trying to find a way of using an Exim filter to deliver a message
along with an error to the original sender. Instead of using "fail text
"This message contains an attachment of unsafe type" etc etc, I am using
"deliver $return_path" but this basically sends the message straight
back to the sender and gives them no warning, nor does it stop the
attachment from doing harm.

How could this be done? Any help is greatly appreciated.

Regards
Richard