diff --git a/plugins/kolab_delegation/kolab_delegation.php b/plugins/kolab_delegation/kolab_delegation.php
index 717aaeb5..a9ccf464 100644
--- a/plugins/kolab_delegation/kolab_delegation.php
+++ b/plugins/kolab_delegation/kolab_delegation.php
@@ -350,15 +350,15 @@ class kolab_delegation extends rcube_plugin
 
         // Delegate delete
         if ($this->rc->action == 'plugin.delegation-delete') {
-            $id      = rcube_utils::get_input_value('id', rcube_utils::INPUT_GPC);
-            $success = $engine->delegate_delete($id, (bool) rcube_utils::get_input_value('acl', rcube_utils::INPUT_GPC));
+            $id    = rcube_utils::get_input_value('id', rcube_utils::INPUT_GPC);
+            $error = $engine->delegate_delete($id, (bool) rcube_utils::get_input_value('acl', rcube_utils::INPUT_GPC));
 
-            if ($success) {
+            if (!$error) {
                 $this->rc->output->show_message($this->gettext('deletesuccess'), 'confirmation');
                 $this->rc->output->command('plugin.delegate_save_complete', array('deleted' => $id));
             }
             else {
-                $this->rc->output->show_message($this->gettext('deleteerror'), 'error');
+                $this->rc->output->show_message($this->gettext($error), 'error');
             }
         }
         // Delegate add/update
@@ -369,23 +369,23 @@ class kolab_delegation extends rcube_plugin
             // update
             if ($id) {
                 $delegate = $engine->delegate_get($id);
-                $success  = $engine->delegate_acl_update($delegate['uid'], $acl);
+                $error    = $engine->delegate_acl_update($delegate['uid'], $acl);
 
-                if ($success) {
+                if (!$error) {
                     $this->rc->output->show_message($this->gettext('updatesuccess'), 'confirmation');
                     $this->rc->output->command('plugin.delegate_save_complete', array('updated' => $id));
                 }
                 else {
-                    $this->rc->output->show_message($this->gettext('updateerror'), 'error');
+                    $this->rc->output->show_message($this->gettext($error), 'error');
                 }
             }
             // new
             else {
                 $login    = rcube_utils::get_input_value('newid', rcube_utils::INPUT_GPC);
                 $delegate = $engine->delegate_get_by_name($login);
-                $success  = $engine->delegate_add($delegate, $acl);
+                $error    = $engine->delegate_add($delegate, $acl);
 
-                if ($success) {
+                if (!$error) {
                     $this->rc->output->show_message($this->gettext('createsuccess'), 'confirmation');
                     $this->rc->output->command('plugin.delegate_save_complete', array(
                         'created' => $delegate['ID'],
@@ -393,7 +393,7 @@ class kolab_delegation extends rcube_plugin
                     ));
                 }
                 else {
-                    $this->rc->output->show_message($this->gettext('createerror'), 'error');
+                    $this->rc->output->show_message($this->gettext($error), 'error');
                 }
             }
         }
diff --git a/plugins/kolab_delegation/kolab_delegation_engine.php b/plugins/kolab_delegation/kolab_delegation_engine.php
index eae73477..bacd35bf 100644
--- a/plugins/kolab_delegation/kolab_delegation_engine.php
+++ b/plugins/kolab_delegation/kolab_delegation_engine.php
@@ -54,6 +54,8 @@ class kolab_delegation_engine
      *
      * @param string|array $delegate Delegate DN (encoded) or delegate data (result of delegate_get())
      * @param array        $acl      List of folder->right map
+     *
+     * @return string On error returns an error label, on success returns null
      */
     public function delegate_add($delegate, $acl)
     {
@@ -63,18 +65,20 @@ class kolab_delegation_engine
 
         $dn = $delegate['ID'];
         if (empty($delegate) || empty($dn)) {
-            return false;
+            return 'createerror';
         }
 
         $list = $this->list_delegates();
-
-        // add delegate to the list
         $list = array_keys((array)$list);
         $list = array_filter($list);
-        if (!in_array($dn, $list)) {
-            $list[] = $dn;
+
+        if (in_array($dn, $list)) {
+            return 'delegationexisterror';
         }
-        $list = array_map(array('kolab_auth_ldap', 'dn_decode'), $list);
+
+        // add delegate to the list
+        $list[] = $dn;
+        $list   = array_map(array('kolab_auth_ldap', 'dn_decode'), $list);
 
         // update user record
         $result = $this->user_update_delegates($list);
@@ -84,7 +88,7 @@ class kolab_delegation_engine
             $this->delegate_acl_update($delegate['uid'], $acl);
         }
 
-        return $result;
+        return $result ? null : 'createerror';
     }
 
     /**
@@ -93,6 +97,8 @@ class kolab_delegation_engine
      * @param string $uid    Delegate authentication identifier
      * @param array  $acl    List of folder->right map
      * @param bool   $update Update (remove) old rights
+     *
+     * @return string On error returns an error label, on success returns null
      */
     public function delegate_acl_update($uid, $acl, $update = false)
     {
@@ -119,8 +125,6 @@ class kolab_delegation_engine
                 $storage->delete_acl($folder_name, $uid);
             }
         }
-
-        return true;
     }
 
     /**
@@ -128,6 +132,8 @@ class kolab_delegation_engine
      *
      * @param string $dn      Delegate DN (encoded)
      * @param bool   $acl_del Enable ACL deletion on delegator folders
+     *
+     * @return string On error returns an error label, on success returns null
      */
     public function delegate_delete($dn, $acl_del = false)
     {
@@ -136,7 +142,7 @@ class kolab_delegation_engine
         $user     = $this->user();
 
         if (empty($delegate) || !isset($list[$dn])) {
-            return false;
+            return 'deleteerror';
         }
 
         // remove delegate from the list
@@ -153,7 +159,7 @@ class kolab_delegation_engine
             $this->delegate_acl_update($delegate['uid'], array(), true);
         }
 
-        return $result;
+        return $result ? null : 'deleteerror';
     }
 
     /**
diff --git a/plugins/kolab_delegation/localization/en_US.inc b/plugins/kolab_delegation/localization/en_US.inc
index 94ceb9e7..cbf66e6f 100644
--- a/plugins/kolab_delegation/localization/en_US.inc
+++ b/plugins/kolab_delegation/localization/en_US.inc
@@ -35,6 +35,7 @@ $labels['updatesuccess'] = 'The delegate was successfully updated.';
 $labels['updateerror'] = 'Could not update delegate.';
 $labels['createsuccess'] = 'The delegate was successfully added.';
 $labels['createerror'] = 'Could not add delegate.';
+$labels['delegationexisterror'] = 'Specified user already is your delegate.';
 
 $labels['arialabeldelegatedelete'] = 'Delegate deletion dialog';
 $labels['arialabeldelegateform'] = 'Delegate properties form';