diff --git a/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryMessagingNetwork.kt b/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryMessagingNetwork.kt
index b297295d36..65f22209f9 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryMessagingNetwork.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryMessagingNetwork.kt
@@ -203,6 +203,7 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
 
         protected inner class InnerState {
             val handlers: MutableList<Handler> = ArrayList()
+            val pendingRedelivery = LinkedList<MessageTransfer>()
         }
 
         protected val state = ThreadBox(InnerState())
@@ -222,9 +223,16 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
 
         override fun addMessageHandler(topic: String, executor: Executor?, callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration {
             check(running)
-            return state.locked {
-                Handler(executor, topic, callback).apply { handlers.add(this) }
+            val (handler, items) = state.locked {
+                val handler = Handler(executor, topic, callback).apply { handlers.add(this) }
+                val items = ArrayList(pendingRedelivery)
+                pendingRedelivery.clear()
+                Pair(handler, items)
             }
+            for (it in items) {
+                send(it.message, handle)
+            }
+            return handler
         }
 
         override fun removeMessageHandler(registration: MessageHandlerRegistration) {
@@ -282,7 +290,12 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
                 val h = handlers.filter { if (it.topic.isBlank()) true else transfer.message.topic == it.topic }
 
                 if (h.isEmpty()) {
-                    loggerFor<InMemoryMessagingNetwork>().warn("No handlers for message ${transfer}")
+                    // Got no handlers for this message yet. Keep the message around and attempt redelivery after a new
+                    // handler has been registered. The purpose of this path is to make unit tests that have multi-threading
+                    // reliable, as a sender may attempt to send a message to a receiver that hasn't finished setting
+                    // up a handler for yet. Most unit tests don't run threaded, but we want to test true parallelism at
+                    // least sometimes.
+                    pendingRedelivery.add(transfer)
                     return null
                 }