Carried over contracts.

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

@@ -0,0 +1,16 @@
+namespace MessengerApi.Contracts
+{
+    /// <summary>
+    /// Outbox type of message. A server-app will treat this as a response. A client app will treat this as a request.
+    /// </summary>
+    public class OutboxMessage
+    {
+        public Guid? ToUserId { get; set; }
+
+        public string PayloadType { get; set; }
+
+        public string Payload { get; set; }
+
+        public int? LifespanInSeconds { get; set; }
+    }
+}