diff --git a/42.md b/42.md
index 53af82f..f06c633 100644
--- a/42.md
+++ b/42.md
@@ -22,6 +22,8 @@ A relay may want to require clients to authenticate to access restricted resourc
 
 Clients that want to force an authentication in any event can add a `requires_auth_by` with array of authorized pubkeys to the base event object. 
 
+Relays MUST authenticate connections with at least one of the pubkeys listed in `requires_auth_by` to return events.
+
 ```jsonc
 {
   "id": <32-bytes lowercase hex-encoded sha256 of the serialized event data>,