[Exim] System Filter question

I've set up the system filter below:

if $recipients_count is above 100 and
      $return_path does not contain "mjl@???"
      then
      fail text "Too many recipients. Maximum 100 allowed"
endif

I had previously been using the max_recipients option, but this one
particular user, wants to be able to send email to all of his
customers....and he's not quite sophisticated enough to use a
listserv. So I commented the max_recipients option, and set up this
system filter based on an example I saw in the mailing list archive.

It seems to work perfectly fine....If I try to send a message with
more than 100 recipients it gets rejected as it should. But I
noticed this morning that someone received a spam message that was
bcc'd to a huge list of people. And in case you're wondering, no
"mjl@???" was not in the return path.

Are there some conditions where a system filter won't be applied to a message?