diff --git a/Build/MQTTnet.nuspec b/Build/MQTTnet.nuspec
index e2bfb867..afcf1e8d 100644
--- a/Build/MQTTnet.nuspec
+++ b/Build/MQTTnet.nuspec
@@ -22,6 +22,7 @@
 * [Server] Session takeover and keep alive timeout are now properly set in DISCONNECT packet.
 * [Server] Fixed bug in PubRel packet generation (MQTTv5 only).
 * [Server] Improved message processing performance (+ ~5%).
+* [Server] Fix wrong usage of client session items for undelivered messages.
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2020</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin Blazor</tags> 
diff --git a/Source/MQTTnet/MqttApplicationMessageReceivedEventArgs.cs b/Source/MQTTnet/MqttApplicationMessageReceivedEventArgs.cs
index 6fbaa933..ed46305c 100644
--- a/Source/MQTTnet/MqttApplicationMessageReceivedEventArgs.cs
+++ b/Source/MQTTnet/MqttApplicationMessageReceivedEventArgs.cs
@@ -2,7 +2,7 @@
 
 namespace MQTTnet
 {
-    public class MqttApplicationMessageReceivedEventArgs : EventArgs
+    public sealed class MqttApplicationMessageReceivedEventArgs : EventArgs
     {
         public MqttApplicationMessageReceivedEventArgs(string clientId, MqttApplicationMessage applicationMessage)
         {
diff --git a/Source/MQTTnet/Server/MqttClientSessionsManager.cs b/Source/MQTTnet/Server/MqttClientSessionsManager.cs
index 0dc95190..18b529e4 100644
--- a/Source/MQTTnet/Server/MqttClientSessionsManager.cs
+++ b/Source/MQTTnet/Server/MqttClientSessionsManager.cs
@@ -287,21 +287,21 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                var sender = queuedApplicationMessage.Sender;
-                var senderClientId = sender?.ClientId ?? _options.ClientId;
+                var clientConnection = queuedApplicationMessage.Sender;
+                var senderClientId = clientConnection?.ClientId ?? _options.ClientId;
                 var applicationMessage = queuedApplicationMessage.ApplicationMessage;
 
                 var interceptor = _options.ApplicationMessageInterceptor;
                 if (interceptor != null)
                 {
-                    var interceptorContext = await InterceptApplicationMessageAsync(interceptor, sender, applicationMessage).ConfigureAwait(false);
+                    var interceptorContext = await InterceptApplicationMessageAsync(interceptor, clientConnection, applicationMessage).ConfigureAwait(false);
                     if (interceptorContext != null)
                     {
                         if (interceptorContext.CloseConnection)
                         {
-                            if (sender != null)
+                            if (clientConnection != null)
                             {
-                                await sender.StopAsync(MqttDisconnectReasonCode.NormalDisconnection).ConfigureAwait(false);
+                                await clientConnection.StopAsync(MqttDisconnectReasonCode.NormalDisconnection).ConfigureAwait(false);
                             }
                         }
 
@@ -343,7 +343,8 @@ namespace MQTTnet.Server
                         return;
                     }
 
-                    await undeliveredMessageInterceptor.InterceptApplicationMessagePublishAsync(new MqttApplicationMessageInterceptorContext(senderClientId, sender?.Session?.Items, applicationMessage));
+                    // The delegate signature is the same as for regular message interceptor. So the call is fine and just uses a different interceptor.
+                    await InterceptApplicationMessageAsync(undeliveredMessageInterceptor, clientConnection, applicationMessage).ConfigureAwait(false);
                 }
             }
             catch (OperationCanceledException)
@@ -427,12 +428,12 @@ namespace MQTTnet.Server
             }
         }
 
-        async Task<MqttApplicationMessageInterceptorContext> InterceptApplicationMessageAsync(IMqttServerApplicationMessageInterceptor interceptor, MqttClientConnection senderConnection, MqttApplicationMessage applicationMessage)
+        async Task<MqttApplicationMessageInterceptorContext> InterceptApplicationMessageAsync(IMqttServerApplicationMessageInterceptor interceptor, MqttClientConnection clientConnection, MqttApplicationMessage applicationMessage)
         {
             string senderClientId;
             IDictionary<object, object> sessionItems;
 
-            var messageIsFromServer = senderConnection == null;
+            var messageIsFromServer = clientConnection == null;
             if (messageIsFromServer)
             {
                 senderClientId = _options.ClientId;
@@ -440,8 +441,8 @@ namespace MQTTnet.Server
             }
             else
             {
-                senderClientId = senderConnection.ClientId;
-                sessionItems = senderConnection.Session.Items;
+                senderClientId = clientConnection.ClientId;
+                sessionItems = clientConnection.Session.Items;
             }
 
             var interceptorContext = new MqttApplicationMessageInterceptorContext(senderClientId, sessionItems, applicationMessage);