[Exim] $local_part not available in authenticators?

Dear list,

I have a very simple question: Why does this not work:

begin authenticators

fixed_login:

server_condition = ${if ldapauth \
       {user="${lookup ldapdn {ldap:///BASEDN?dn?sub?(mail=$local_part)}"
pass=${quote:$2} \
      ldap://someldapserver.de/}{yes}{no}}

Taking a look at the query that is received by the LDAP server:

conn=2494 op=1 SRCH base="dc=X,dc=Y,dc=Z,o=world" scope=2 deref=0
filter="(?=undefined)"

I could understand if this would show as filter="(mail=undefined)" but what
I would expect was much more

filter="(mail=mylocalpart)"

Any hints are welcome.

Torsten