[exim] System_filter.exim for spam filtering

Spamassassin scores messages on our server with a header like so.

X-Spam-Level: *****************

With the number of astericks indicating its spam score.

Will this added to system_filter.exim block all messages that score over 15?

if $header_x-spam-level: contains "***************" then
    fail text "This message has been rejected due to spam content.\n\"
    seen finish
endif

Matt