[Exim] condition subject rule

hi everybody,

i am running exim 3.53 (debian woody) and i installed amavis-ng and
spamassassin all is working just fine.

now i want to give users the possibilty to send mails with a specified
subject not be scanned by spamassassin.

i read the exim specification doc but i dont get it :(

the default condition for spamassasin is:
condition = "${if and { {!def:h_X-Spam-Flag:} {!eq
{$received_protocol}{spam-scanned}} {!eq {$received_protocol}{no-scan}}
{!eq {$received_protocol}{local}} } {1}{0}}"

now i tried {!match {message_headers}{nospam}}

but i doesent work.

any ideas?

greets Kevin