[exim] Check for header in ACL

Hello everybody.

I want to check incoming mail for a specific header which is added
without problems on another server: "X-ABC-Greylist: 1".

I read the dcoumentation about "def:header_<header name>:" and created a
very small acl_smtp_data:

acl_smtp_data:
        warn condition = ${if def:header_X-ABC-Greylist:}
             log_message  = Accepted greylisted mail sender \
$sender_address ip $sender_host_address -  $local_part@$domain

        accept

I've also tried various "if match" conditions with $message_headers
without any luck. How would I check correctly for the existence of this
header?

Thanks,
Kai