[Exim] Exim 4.24 final catch router config

Hi All,

We have Exim 4.24 setup here as our MTA it works great except for one thing.

We want to have a final router setup that catches anything that was
missed by the other routers that then uses a transport we have defined
to take care of that.

We have tried the following router config but it doesn't make a match on
anything for some reason. We have looked through the documentation (it's
pretty good so well done) and will keep going through but haven't found
anything as yet.

---------------router config snippet ----------------------

non_exist:
driver = accept
transport = non_exist_reply
no_verify

-------------------end snippet-----------------------------

----------------transport snippet -------------------------

non_exist_reply:
   driver = autoreply
   user = mail
   to = $sender_address
   subject = User does not exist
   text = You sent mail to $local_part. That's not a valid user here. \
          The subject was: $subject.

---------------------end snippet---------------------------

The router is placed last out of the routers and the transport is last
as well.

Any help or insight much appreciated.

Regards,
Pete