I changed a few lines of code in the 3 z-push backend files which seems to work ok for both 2 and 3 level domain names. This is in the function "ldap_authenticate" of tasks.php, calendar.php and vcarddir.php.
Original first few lines:
$et = strrpos($username, "@");
$p = $this->lastIndexOf($username, ".");
$et_dom = substr($username, $et + 1, $p - strlen($username));
$dot_ext = substr($username, $p + 1, strlen($username));
$username = substr($username, 0, $et);
$ldaprdn = "uid=$username,ou=people,dc=" . $et_dom . ",dc=" . $dot_ext;
Changed to this:
$et = strrpos($username, "@");
$et_dom = substr($username, $et + 1);
$et_dom = str_replace(".",",dc=",$et_dom);
$username = substr($username, 0, $et);
$ldaprdn = "uid=$username,ou=people,dc=" . $et_dom;
Maybe someone knows if this is good enough or whether other changes are necessary to correct the issue.