acl_m0 trick

I think this is more of an exim thing, but I've tried using the acl_m0
trick that's mentioned in the README for sa-exim but the acl never seems
to be set, which means that email sent to abuse/postmaster is still
being rejected, and local mail is being scanned which I don't want.

The lines I have are:

  accept hosts = :
           set acl_m0 = do-not-scan

  accept local_parts = postmaster : abuse
         domains = +local_domains
         set acl_m0 = do-not-rej

  accept hosts = +relay_from_hosts
        set acl_m0 = do-not-scan

  accept authenticated = *
        set acl_m0 = do-not-scan

Any ideas why these wouldn't be set? Should I be using acl_cx instead of acl_mx?