diff --git a/shared/lib/Mail/Service/ServiceEntityMutableInterface.php b/shared/lib/Mail/Service/ServiceEntityMutableInterface.php
index 238bdb2..0984ba0 100644
--- a/shared/lib/Mail/Service/ServiceEntityMutableInterface.php
+++ b/shared/lib/Mail/Service/ServiceEntityMutableInterface.php
@@ -56,12 +56,19 @@ interface ServiceEntityMutableInterface {
     /**
      * Modifies an existing entity
      *
+     * The returned entity's identifier is authoritative and may differ from
+     * the target identifier. Providers without in-place update support may
+     * implement this as an append-and-replace (or equivalent create-then-
+     * destroy) operation, in which case the target identifier is superseded
+     * and no longer resolvable; callers must persist the returned identifier
+     * rather than assume the target remains valid.
+     *
      * @since 2025.05.01
      *
      * @param EntityIdentifier $target Target entity identifier
      * @param MessagePropertiesMutableInterface $properties Entity properties to update
      *
-     * @return EntityBaseInterface Modified entity
+     * @return EntityBaseInterface Modified entity, with its own authoritative identifier
      */
     public function entityModify(EntityIdentifier $target, MessagePropertiesMutableInterface $properties): EntityBaseInterface;