на сайте документации exim
http://www.exim.org/exim-html-current/doc/html/spec_html/ch34.html
указанн пример
login:
driver = plaintext
public_name = LOGIN
server_prompts = Username:: : Password::
server_condition = ${if and{{ \
!eq{}{$auth1} }{ \
ldapauth{user="cn=${quote_ldap_dn:$auth1},ou=people,o=example.org" \
pass=${quote:$auth2} \
ldap://ldap.example.org/} }} }
server_set_id = uid=$auth1,ou=people,o=example.org
смотреть в сторону ldapdn ?