diff --git a/doc/sample-kolab.conf b/doc/sample-kolab.conf
--- a/doc/sample-kolab.conf
+++ b/doc/sample-kolab.conf
@@ -43,6 +43,10 @@
       "specialChars" : "<>#&%!?.,;*/+-=[]{}()",
       "7bitPasswordEnforcement": 1
        }
+; should we enforce unique user identities (UID) across multiple domains? This is disabled by default (false).
+; if it is enabled, it is possible to login just with the UID without using Cyrus canonification,
+; and when creating a user in the WAP the uniqueness of the UID across domains is enforced
+unique_uid_across_domains = false
 
 [ldap]
 uri = ldap://ldap.klab.cc
diff --git a/lib/Auth.php b/lib/Auth.php
--- a/lib/Auth.php
+++ b/lib/Auth.php
@@ -451,6 +451,11 @@
         return $this->auth_instance()->user_find_by_attribute($attribute);
     }
 
+    public function user_find_by_attribute_per_domain($domain, $attribute)
+    {
+        return $this->auth_instance($domain)->user_find_by_attribute($attribute);
+    }
+
     public function user_info($userdata, $attributes = array('*'))
     {
         return $this->auth_instance()->user_info($userdata, $attributes);
diff --git a/lib/api/kolab_api_service_form_value.php b/lib/api/kolab_api_service_form_value.php
--- a/lib/api/kolab_api_service_form_value.php
+++ b/lib/api/kolab_api_service_form_value.php
@@ -840,21 +840,50 @@
 
             $orig_uid = $uid;
             $auth     = Auth::get_instance($_SESSION['user']->get_domain());
-
+            $conf     = Conf::get_instance();
+
+            $all_domains = array();
+           
+            if ($conf->get('unique_uid_across_domains') == "true") {
+                $domainslist = $auth->list_domains();
+                foreach ($domainslist['list'] as $domainobj) {
+                    $domain = $domainobj['associateddomain'];
+                    if (is_array($domain)) {
+                        $domain = $domain[0];
+                    }
+                    $all_domains[] = $domain;
+                }
+            } else {
+                $all_domains[] = $_SESSION['user']->get_domain();
+            }
+    
             $x = 2;
-            while (($user_found = $auth->user_find_by_attribute(array('uid' => $uid)))) {
-                if (!empty($postdata['id'])) {
-                    $user_found_dn          = key($user_found);
-                    $user_found_unique_attr = $this->unique_attribute_value($user_found_dn);
-                    //console("user with uid $uid found", $user_found_unique_attr);
-                    if ($user_found_unique_attr == $postdata['id']) {
-                        //console("that's us.");
-                        break;
+            while (true) {
+                $user_found = null;
+                foreach ($all_domains as $domain) {
+                    $user_found = $auth->user_find_by_attribute_per_domain($domain, array('uid' => $uid));
+
+                    if (!empty($postdata['id'])) {
+                        $user_found_dn          = key($user_found);
+                        $user_found_unique_attr = $this->unique_attribute_value($user_found_dn);
+                        //console("user with uid $uid found", $user_found_unique_attr);
+                        if ($user_found_unique_attr == $postdata['id']) {
+                            //console("that's us.");
+                            $user_found = null;
+                            break; // foreach domain
+                        }
+                    }
+                    if ($user_found) {
+                        break; // foreach domain
                     }
                 }
 
-                $uid = $orig_uid . $x;
-                $x++;
+                if ($user_found) {
+		    $uid = $orig_uid . $x;
+                    $x++;
+                } else {
+                    break; // while true
+                }
             }
 
             return $uid;
diff --git a/lib/api/kolab_api_service_user.php b/lib/api/kolab_api_service_user.php
--- a/lib/api/kolab_api_service_user.php
+++ b/lib/api/kolab_api_service_user.php
@@ -88,6 +88,33 @@
         Log::trace("user_add()", $attributes);
 
         $auth   = Auth::get_instance();
+        $conf   = Conf::get_instance();
+
+        # validate if the uid is unique inside this domain, or across multiple domains
+        $all_domains = array();
+
+        if ($conf->get('unique_uid_across_domains') == "true") {
+            $domainslist = $auth->list_domains();
+            foreach ($domainslist['list'] as $domainobj) {
+                $domain = $domainobj['associateddomain'];
+                if (is_array($domain)) {
+                    $domain = $domain[0];
+                }
+                $all_domains[] = $domain;
+            }
+        } else {
+            $all_domains[] = $_SESSION['user']->get_domain();
+        }
+   
+        $user_found = null;
+        foreach ($all_domains as $domain) {
+            $user_found = $auth->user_find_by_attribute_per_domain($domain, array('uid' => $attributes['uid']));
+            if ($user_found) {
+                throw new Exception(kolab_api_controller::translate("error").": ".
+                   kolab_api_controller::translate("user.add.duplicateuid", $attributes['uid']));
+            }
+        }
+
         $result = $auth->user_add($attributes, $postdata['type_id']);
 
         if ($result) {
@@ -231,4 +258,38 @@
         return false;
     }
 
+    /**
+     * Find user and return his domain.
+     * If the search returns only one record we'll return the domain of the user.
+     * 
+     * @param array $get   GET parameters
+     * @param array $post  POST parameters
+     *
+     * @return array|bool User attributes, False on error
+     */
+    public function user_get_domain($get, $post)
+    {
+        $auth       = Auth::get_instance();
+        $all_domains   = $auth->list_domains();
+        $all_domains   = $all_domains['list'];
+        $exists_in_domain = "";
+
+        foreach ($all_domains as $domainobj) {
+            $domain = $domainobj['associateddomain'];
+            if (is_array($domain)) {
+                $domain = $domain[0];
+            }
+            if ($auth->user_find_by_attribute_per_domain($domain, array('uid' => $get['id']))) {
+                if (!empty($exists_in_domain)) {
+                    // there are multiple users with the same userid in different domains
+                    return false;
+                }
+                $exists_in_domain = $domain;
+            }
+        }
+        if (empty($exists_in_domain)) {
+            return false;
+        }
+        return array('domain' => $exists_in_domain);
+    }
 }
diff --git a/lib/kolab_client_task.php b/lib/kolab_client_task.php
--- a/lib/kolab_client_task.php
+++ b/lib/kolab_client_task.php
@@ -210,6 +210,27 @@
                 $login['username'] = trim($login['username']);
                 $login['domain']   = trim($login['domain']);
 
+                if ($this->config->get('kolab', 'unique_uid_across_domains') == "true") {
+                    $dirman = $this->config->get('ldap', 'bind_dn');
+                    if (strpos($login['username'], '@') === false && $login['username'] != $dirman) {
+                        $templogin = $this->api->login($dirman, $this->config->get('ldap', 'bind_pw'));
+
+                        if ($temptoken = $templogin->get('session_token')) {
+                            $this->api->set_session_token($temptoken);
+                        }
+
+                        if (($result = $this->api->get('user.get_domain', array('id' => $login['username']))) !== false) {
+                            $result = $result->get();
+                            if (array_key_exists('domain', $result)) {
+                                $login['domain'] = $result['domain'];
+                            }
+                        }
+
+                        // reset the api connection
+                        $this->api_init();
+                    }
+                }
+
                 $result = $this->api->login($login['username'], $login['password'], $login['domain'], true);
 
                 if ($token = $result->get('session_token')) {
diff --git a/lib/locale/de_DE.php b/lib/locale/de_DE.php
--- a/lib/locale/de_DE.php
+++ b/lib/locale/de_DE.php
@@ -353,6 +353,7 @@
 $LANG['type.user'] = 'Benutzer';
 
 $LANG['user.add'] = 'Benutzer hinzufügen';
+$LANG['user.add.duplicateuid'] = 'Die eindeutige Identität (UID) $1 ist schon vergeben.';
 $LANG['user.add.success'] = 'Benutzer erfolgreich hinzugefügt.';
 $LANG['user.alias'] = 'Sekundäre Email-Adresse';
 $LANG['user.astaccountallowedcodec'] = 'Erlaubte(r) Codec(s)';
diff --git a/lib/locale/en_US.php b/lib/locale/en_US.php
--- a/lib/locale/en_US.php
+++ b/lib/locale/en_US.php
@@ -351,6 +351,7 @@
 $LANG['type.user'] = 'User';
 
 $LANG['user.add'] = 'Add User';
+$LANG['user.add.duplicateuid'] = 'The unique identity (UID) $1 is already in use.';
 $LANG['user.add.success'] = 'User created successfully.';
 $LANG['user.alias'] = 'Secondary Email Address(es)';
 $LANG['user.astaccountallowedcodec'] = 'Allowed codec(s)';