LDAP authentication failed

I have saslauthd service installed on my Ubuntu (sudo apt-get install sasl2-bin). I configured saslauth to use LDAP for user authentication and /etc/default/saslauthd now looks like this:

# /etc/default/saslauthd
START=yes
DESC="SASL Authentication Daemon"
NAME="saslauthd"
MECHANISMS="ldap"
MECH_OPTIONS=""
THREADS=5
OPTIONS="-c -m /var/run/saslauthd"

Next I configured LDAP binding in /etc/saslauthd.conf.

# /etc/saslauthd.conf
ldap_servers: ldap://192.168.1.100:1389
ldap_filter: (mail=%u)
ldap_search_base: o=users

I would like to write a simple nodejs-based LDAP server that would authenticate user based on the (mail=%u) filter. For start it would be great to always say success. I decided to use ldapjs for this (here) - no decurity is needed because the ldap service will run on localhost.

var ldap = require('ldapjs');
var server = ldap.createServer();

server.search('o=users', function(req, res, next) {
  console.log("I see this log!");
  // The idea is to look for a user in mongodb
  res.end();
  return next();
});

server.listen(1389, function() {
  console.log('LDAP server up at: %s', server.url);
});

I work with LDAP for the first time and I guess I don't have all the attributes for this server to work :). The server always fails to authenticate. The log file says

Sep  2 12:33:14 ubuntu saslauthd[19907]: Entry not found ((mail=user1)).
Sep  2 12:33:14 ubuntu saslauthd[19907]: Authentication failed for user1: User not found (-6)
Sep  2 12:33:14 ubuntu saslauthd[19907]: do_auth         : auth failure: [user=user1] [service=] [realm=] [mech=ldap] [reason=Unknown]

The res.end(); call should always return success but it seems that the server should respond differently or smth.

The question is how to write such server that would working with saslauthd. Thanks!s