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
@@ -1276,6 +1276,11 @@
             }
         }
 
+        // Check if addresses are not already in use
+        if ($validation_type == self::VALIDATE_EXTENDED) {
+            $this->_email_addresses_in_use($value, 'mail', $postdata);
+        }
+
         return 'OK';
     }