[Exim] badtrans virus

Hi Guys,

I've added a bit to my filter to try and stop the badtrans worm.
This is what i have in my filter.

if $header_from: contains <_ and $sender_helo_name is aol.com then
    logfile /var/log/exim/exim_newvirus
    logwrite "$tod_log $header_from: sent the new virus to $header_to:"
    if $sender_address_local_part begins _
then
  fail
   text "Your machine appears to be affected with the W32.Badtrans.B@mm worm.\
      \n As a result your e-mail has been returned.\
      \n Please visit
http://www.symantec.com/avcenter/venc/data/w32.badtrans.b@mm.removal.tool.ht
ml\
      \n for instructions on how to remove this virus."
endif
endif

it's loggging the viruses quite happily, but it doesnt appear to be
cancelling the e-mail.
Have i done something wrong?