[Exim] :fail: message?

I have set up my mailing lists like this:

I have a directory with a file for each list with entries for the
email addresses, and I have a file that has all the addresses and I
allow anyone on any list to post to any list (basically this stops
spam/advertising getting through which is all I really want to do).

My routers look like this:

lists_request:
driver = redirect
local_part_suffix = -request:-subscribe:-unsubscribe:-on:-off:-info
data = listmaster
no_more

lists_post:
   driver = redirect
#  require_files /Users/exim/exim/lists/$local_part
   senders = ${if exists {/Users/exim/exim/lists/$local_part} \
                 {lsearch;/Users/exim/exim/canpost} \
                 fail \
              }
   modemask = 113
   file = /Users/exim/exim/lists/$local_part
   forbid_pipe
   forbid_file
   errors_to = $local_part-request
   no_more

lists_closed:
driver = redirect
require_files = /Users/exim/exim/lists/$local_part
allow_fail
data = :fail: mail $local_part-request for permission to post to
this mailing list
no_more

If a non-sender posts to the list, I get the log message:

2002-06-12 16:07:39 H=localhost [127.0.0.1]
F=<someone@???> rejected RCPT <testlist@zany>: mail
testlist-request for permission to post to this mailing list

But the SMTP server says:

550 unknown user

Why doesn't the fail message make it through to the SMTP server?
Does it only get used for bounce messages?

Thanks,
    Peter.

--
<http://www.interarchy.com/> <http://download.interarchy.com/>