diff --git a/node-api/src/main/kotlin/net/corda/nodeapi/internal/protonwrapper/engine/ConnectionStateMachine.kt b/node-api/src/main/kotlin/net/corda/nodeapi/internal/protonwrapper/engine/ConnectionStateMachine.kt
index 5d542e1964..38062c20b9 100644
--- a/node-api/src/main/kotlin/net/corda/nodeapi/internal/protonwrapper/engine/ConnectionStateMachine.kt
+++ b/node-api/src/main/kotlin/net/corda/nodeapi/internal/protonwrapper/engine/ConnectionStateMachine.kt
@@ -81,7 +81,7 @@ internal class ConnectionStateMachine(private val serverMode: Boolean,
     val connection: Connection
     private val transport: Transport
     private val id = UUID.randomUUID().toString()
-    private var session: Session? = null
+    private val sessionState = SessionState()
     /**
      * Key is message topic and value is the list of messages
      */
@@ -144,7 +144,7 @@ internal class ConnectionStateMachine(private val serverMode: Boolean,
         logInfoWithMDC("Connection local open ${connection.prettyPrint}")
         val session = connection.session()
         session.open()
-        this.session = session
+        sessionState.init(session)
         for (target in messageQueues.keys) {
             getSender(target)
         }
@@ -206,7 +206,7 @@ internal class ConnectionStateMachine(private val serverMode: Boolean,
             }
             // shouldn't happen, but cleanup any stranded items
             transport.context = null
-            session = null
+            sessionState.close()
             receivers.clear()
             senders.clear()
         } else {
@@ -287,7 +287,7 @@ internal class ConnectionStateMachine(private val serverMode: Boolean,
 
     private fun getSender(target: String): Sender {
         if (!senders.containsKey(target)) {
-            val sender = session!!.sender(UUID.randomUUID().toString())
+            val sender = sessionState.session!!.sender(UUID.randomUUID().toString())
             sender.source = Source().apply {
                 address = target
                 dynamic = false
@@ -315,9 +315,9 @@ internal class ConnectionStateMachine(private val serverMode: Boolean,
 
     override fun onSessionFinal(event: Event) {
         val session = event.session
-        logDebugWithMDC { "Session final $session" }
-        if (session == this.session) {
-            this.session = null
+        logDebugWithMDC { "Session final for: $session" }
+        if (session == sessionState.session) {
+            sessionState.close()
 
             // If TRANSPORT_CLOSED event was already processed, the 'transport' in all subsequent events is set to null.
             // There is, however, a chance of missing TRANSPORT_CLOSED event, e.g. when disconnect occurs before opening remote session.
@@ -451,8 +451,15 @@ internal class ConnectionStateMachine(private val serverMode: Boolean,
             logDebugWithMDC { "Sender delivery confirmed tag ${delivery.tag.toHexString()}" }
             val ok = delivery.remotelySettled() && delivery.remoteState == Accepted.getInstance()
             val sourceMessage = delivery.context as? SendableMessageImpl
-            unackedQueue.remove(sourceMessage)
-            sourceMessage?.doComplete(if (ok) MessageStatus.Acknowledged else MessageStatus.Rejected)
+            if (sourceMessage != null) {
+                unackedQueue.remove(sourceMessage)
+                val status = if (ok) MessageStatus.Acknowledged else MessageStatus.Rejected
+                logDebugWithMDC { "Setting status as: $status to message with wire uuid: " +
+                        "${sourceMessage.applicationProperties[MESSAGE_ID_KEY]}" }
+                sourceMessage.doComplete(status)
+            } else {
+                logDebugWithMDC { "Source message not found on delivery context" }
+            }
             delivery.settle()
         }
     }
@@ -500,11 +507,22 @@ internal class ConnectionStateMachine(private val serverMode: Boolean,
         msg.buf = encoded
         val messageQueue = messageQueues.getOrPut(msg.topic, { LinkedList() })
         messageQueue.offer(msg)
-        if (session != null) {
-            val sender = getSender(msg.topic)
-            transmitMessages(sender)
-        } else {
-            logInfoWithMDC("Session been closed already")
+        when (sessionState.value) {
+            SessionState.Value.ACTIVE -> {
+                val sender = getSender(msg.topic)
+                transmitMessages(sender)
+            }
+            SessionState.Value.UNINITIALIZED -> {
+                // This is pretty normal as on Connection Local may not have been received yet
+                // Once it will be received the messages will be sent from the `messageQueues`
+                logDebugWithMDC { "Session has not been open yet" }
+            }
+            SessionState.Value.CLOSED -> {
+                logInfoWithMDC("Session been closed already")
+                // If session been closed then it is too late to send a message, so we flag it as rejected.
+                logDebugWithMDC { "Setting Rejected status to message with wire uuid: ${msg.applicationProperties[MESSAGE_ID_KEY]}" }
+                msg.doComplete(MessageStatus.Rejected)
+            }
         }
     }
 
diff --git a/node-api/src/main/kotlin/net/corda/nodeapi/internal/protonwrapper/engine/SessionState.kt b/node-api/src/main/kotlin/net/corda/nodeapi/internal/protonwrapper/engine/SessionState.kt
new file mode 100644
index 0000000000..7c81e43d62
--- /dev/null
+++ b/node-api/src/main/kotlin/net/corda/nodeapi/internal/protonwrapper/engine/SessionState.kt
@@ -0,0 +1,34 @@
+package net.corda.nodeapi.internal.protonwrapper.engine
+
+import org.apache.qpid.proton.engine.Session
+
+/**
+ * In addition to holding the `Session` also tracks the state of it.
+ */
+internal class SessionState {
+
+    enum class Value {
+        UNINITIALIZED,
+        ACTIVE,
+        CLOSED
+    }
+
+    private var _value: Value = Value.UNINITIALIZED
+
+    private var _session: Session? = null
+
+    val value: Value get() = _value
+
+    val session: Session? get() = _session
+
+    fun init(session: Session) {
+        require(value == Value.UNINITIALIZED)
+        _value = Value.ACTIVE
+        _session = session
+    }
+
+    fun close() {
+        _value = Value.CLOSED
+        _session = null
+    }
+}
\ No newline at end of file