[Exim] LDAP check during smtp dialogue

Hi all,

this my config:

receiver_verify = true

local_not_exist:
    condition = "${lookup ldap
{ldap:///c=IT?mail?sub?(&(shortname=$local_part))}{0}{1}}"
    driver = smartuser
    transport = rcpt_not_found_autoreply

But...how can i check recipients during SMTP dialogue and not after
accepting mails?

Thanks,

Riccardo