[Exim] Exim+LDAP,Can't contact LDAP server

Hello.
I want to use Exim+LDAP to verify the valid address.

Exim config file:

acl_check_rcpt:

  deny  message = DISABLED!!!
    domains = +protected_domains
    condition = ${lookup ldap {ldap://SERVER/dc=test,dc=test??sub?(uid=$local_part)}{0}{1}}

If LDAP is down on SERVER exim returns
451 Temporary local problem - please try later


but I want to accept mail if LDAP is down.

Any help is greatly appreciated.