Carried over contracts.

code/MessengerApi.Contracts/Model/Messages/InboxMessage.cs

@@ -0,0 +1,16 @@
+namespace MessengerApi.Contracts
+{
+    /// <summary>
+    /// Message when received is inbox. For server apps, this is request-type of message. For clients, this is a response-type of message.
+    /// </summary>
+    public class InboxMessage
+    {
+        public Guid Id { get; set; }
+
+        public Guid Sender { get; set; }
+
+        public string PayloadType { get; set; }
+
+        public string Payload { get; set; }
+    }
+}