This is my setup on the backup MX, remember these configs might be a different location on your distro. I'm using CentOS 6.3.
/etc/openldap/slapd.conf - append this to bottom of file and change the settings to your own.
syncrepl rid=001
provider=ldap://primarymx.example.co.uk:389
searchbase="dc=example,dc=co,dc=uk"
bindmethod=simple
binddn="cn=Manager,dc=example,dc=co,dc=uk"
credentials=********
schemachecking=on
type=refreshOnly
retry="60 +"
scope=sub
interval=00:00:01:00
attrs="*,+"/etc/postfix/ldap/relay_domains.conf
server_host = 127.0.0.1
server_port = 389
bind = yes
start_tls = no
version = 3
bind_dn = cn=vmail,dc=example,dc=co,dc=uk
bind_pw = ********
search_base = o=domains,dc=example,dc=co,dc=uk
scope = one
#query_filter = (&(objectClass=mailDomain)(|(domainName=%s)(&(enabledService=domainalias)(domainAliasName=%s)))(domainBackupMX=yes)(accountStatus=active)(enabledService=mail)) # this is the old line
query_filter = (&(objectClass=mailDomain)(|(domainName=%s)(&(enabledService=domainalias)(domainAliasName=%s)))(accountStatus=active)(enabledService=mail)) # new config line with domainBackupMX=yes removed.
result_attribute= domainName
debuglevel = 0