[exim] Exim4 RCPT ACL

I have the following rule in an attempt to allow my users to whitelist
forwarders, but if the mail is for the users alias,
the /home/xxxx/.forwarders file obviously doesn't exist.

### Allow users to white list forwarders
  warn
        log_message     = User .forwarders whitelist rule
        domains         = +local_domains
        set acl_m9      = /home/${extract{1}{=}{${lc:
$local_part}}}/.forwarders
        hosts           = ${if exists {$acl_m9}{$acl_m9}}
        set acl_m0      = do-not-reject

So how do I catch things after alias has been resolved?