diff --git a/shared/lib/Mail/Provider/ProviderBaseInterface.php b/shared/lib/Mail/Provider/ProviderBaseInterface.php
index 9e15a4f..592bdb7 100644
--- a/shared/lib/Mail/Provider/ProviderBaseInterface.php
+++ b/shared/lib/Mail/Provider/ProviderBaseInterface.php
@@ -22,6 +22,17 @@ use KTXF\Resource\Provider\ResourceProviderBaseInterface;
 interface ProviderBaseInterface extends ResourceProviderBaseInterface{
 
     public const JSON_TYPE = 'mail:provider';
+
+    /**
+     * Reserved user identifier for system-owned mail services
+     *
+     * Services stored under this user id belong to the tenant itself rather
+     * than any real user (e.g. accounts used to deliver system-generated
+     * messages such as verification codes and password resets).
+     *
+     * @since 2026.07.01
+     */
+    public const USER_SYSTEM = 'system';
     
     /**
      * Finds a service that handles a specific email address