Update 40.md

40.md

@@ -41,6 +41,12 @@ Clients SHOULD use the `supported_nips` field to learn if a relay supports this
 
 Clients MUST ignore events that have expired.
 
+Relay Behavior
+--------------
+
+Relays SHOULD NOT delete an expired message immediately on expiration. Relays that wish to purge expired messages from their DB should allow for a grace period before deletion. For example, this could be 24h after event expiration.
+Relays MAY choose to not delete expired messages.
+
 Suggested Use Cases
 -------------------