[Exim] Blocking based on the subject

I am using a SQL backend that allows users to setup mail blocking based on
sender host,domain,email etc..

But would like to be able to block based on the subject as well, but can't
see a variable to be able to get this in the SQL lookup i.e. line
$sender_address_domain but something line $subject ??

Am I missing something or is this not an option?

Darren