[Exim] Concept of LDAP usage

Dear list,

I am a bit suprised that this is not a FAQ, but maybe I am the only one who
does not get this.

I am trying to set up SMTP auth with LDAP; especially to support the LOGIN
mechanism. (Guess for what MTA ...?)

What confuses me in the first place is that some example (in the Exim spec,
which is the only documentation that I stick to as a lot of the stuff out
there on the net is still 3.x and therefore outdated) use the lookup ldap way
while the specific example for the LOGIN authenticator

(from Section 34.3)

login:
    driver = plaintext
    public_name = LOGIN
    server_prompts = Username:: : Password::
    server_condition = ${if ldapauth \
      {user="cn=${quote_ldap_dn:$1},ou=people,o=example.org" \
      pass=${quote:$2} \
      ldap://ldap.example.org/}{yes}{no}}
    server_set_id = uid=$1,ou=people,o=example.org

is using ldapauth.

So what's happening to my understanding is that it is not trying to look up
the user credentials on the LDAP server but it is trying to log in to LDAP
using the credentials of the person who is trying to send mail with that MTA.

Did I get the concept of LDAP wrong somehow?

Torsten