[Exim] System wide filter question

Hello,

I have a system wide filter that I am using to throw away spam and other email
still being sent to former employees' email addresses. It works quite well and the
junk email cluttering up the queue has been reduced. However, I notice from the
logs that some of this email is still getting through. I chose $message_headers
rather than something specific like $h_To: because I figured it should be able to
get all of the junk. What am I missing and what can I do to completely prevent
these unnecessary emails from getting through?

Here is the message filter file with changed addresses:

-----------------------Begin filter file----------------
# Exim filter
#

if error_message then finish endif

if $message_headers contains user1@??? or
$message_headers contains user2@??? or
$message_headers contains user3@??? or
$message_headers contains user4@??? or
$message_headers contains user5@??? or
$message_headers contains user6@??? or
$message_headers contains user7@??? or
$message_headers contains user8@??? or
$message_headers contains user9@??? or
$message_headers contains user10@??? or
$message_headers contains user11@??? or
$message_headers contains user12@??? or
$message_headers contains user13@??? or
$message_headers contains user14@??? or
$message_headers contains user15@??? or
$message_headers contains user16@??? or
$message_headers contains user17@??? or
$message_headers contains user18@???
then
seen finish
endif
--------------------End filter file-------------------

I left the error stuff in there because the documentation recommended it.

Thanks,

Guru