diff --git a/subscriptionHandler.ts b/subscriptionHandler.ts
index adc254b..4a08c91 100644
--- a/subscriptionHandler.ts
+++ b/subscriptionHandler.ts
@@ -1,9 +1,46 @@
+import Crypto from "crypto";
 import { WS } from "./index";
 
 export interface Subscriber {
   send: (message: string) => void;
 }
 
+export class Mailbox implements Subscriber {
+  // basic
+  id = Crypto.randomUUID();
+  messages = new Set<string>();
+
+  // websocket
+  _ws: WS | undefined;
+
+  get ws(): WS | undefined {
+    return this._ws;
+  }
+
+  set ws(ws: WS) {
+    this._ws = ws;
+    this.sendUnreadMessages();
+  }
+
+  // sending
+  send(message) {
+    if (this._ws) {
+      this._ws.send(message);
+    } else {
+      this.messages.add(message);
+    }
+  }
+
+  sendUnreadMessages() {
+    this.messages.forEach((message) => {
+      if (!this._ws) return;
+
+      this._ws.send(message);
+      this.messages.delete(message);
+    });
+  }
+}
+
 export class SubscriptionMap {
   channelsPerSubscriber = new Map<Subscriber, Set<string>>();
   subscribersPerChannel = new Map<string, Set<Subscriber>>();
diff --git a/websocket-handler.ts b/websocket-handler.ts
index f827e13..5ab0d54 100644
--- a/websocket-handler.ts
+++ b/websocket-handler.ts
@@ -1,7 +1,4 @@
-import {
-  Subscriber,
-  SubscriptionMap,
-} from "./subscriptionHandler";
+import { Subscriber, SubscriptionMap } from "./subscriptionHandler";
 import {
   doIfIsString,
   parseMessage,
@@ -23,11 +20,13 @@ export interface WebSocketMessage {
 
   // mailbox
   requestingMailboxSetup?: boolean;
-  assignedMailboxId?: string;
+  assignedMailboxId?: string; // sent by server
+  requestedMailbox?: string;
 
   // subscribing to channel
   subscribeChannel?: string;
   unsubscribeChannel?: string;
+  subscribed?: string; // sent by server to confirm subscription
 
   //sending message
   messageChannel?: string;
@@ -216,7 +215,7 @@ function confirmSubscription(
   subscriber: Subscriber,
   messageChannel: string
 ): void {
-  const messageObject = {
+  const messageObject: WebSocketMessage = {
     messageChannel,
     subscribed: subscriptionMap.getChannelList(subscriber)?.has(messageChannel)
       ? true