[Exim] antispam with ldap

Hello list,

I have a ldap-directory with valid users, and I only want to accept
users in this directory.

ldapsearch -h ldapsrv -b "ou=DE,o=tirili" -x mail="my.user@???" mail

returns

dn: CN=My User,OU=DE,O=tirili
mail: my.user@???

I thought about a ACL like this

acl_check_rcpt:

   deny   message   = User Unknown
          condition = ${lookup ldap query = ?????????}

But how is the correct syntax for the ldap query ?

Or do you have better Ideas for my accept-only-users-in-ldap-directory ?

Thanks for any help in advance.

Thomas.

--
Thomas Baumann // tirili+list.exim@???