diff --git a/package.yaml b/package.yaml
index b6333b893..9c0bf214a 100644
--- a/package.yaml
+++ b/package.yaml
@@ -29,6 +29,8 @@ executables:
   simplex-messaging:
     source-dirs: src
     main: Main.hs
+    ghc-options:
+      - -threaded
 
 library:
   source-dirs: src
@@ -45,7 +47,6 @@ tests:
 
 ghc-options:
   # - -haddock
-  - -O0
   - -Wall
   - -Wcompat
   - -Werror=incomplete-patterns
diff --git a/src/Env/STM.hs b/src/Env/STM.hs
index f651ca295..378df6232 100644
--- a/src/Env/STM.hs
+++ b/src/Env/STM.hs
@@ -32,27 +32,39 @@ data Env = Env
 
 data Server = Server
   { subscribedQ :: TBQueue (RecipientId, Client),
-    connections :: TVar (Map RecipientId Client)
+    subscribers :: TVar (Map RecipientId Client)
   }
 
 data Client = Client
-  { connections :: TVar (Map RecipientId (Either () ThreadId)),
+  { subscriptions :: TVar (Map RecipientId Sub),
     rcvQ :: TBQueue Signed,
     sndQ :: TBQueue Signed
   }
 
+data SubscriptionThread = NoSub | SubPending | SubThread ThreadId
+
+data Sub = Sub
+  { subThread :: SubscriptionThread,
+    delivered :: TMVar ()
+  }
+
 newServer :: Natural -> STM Server
 newServer qSize = do
   subscribedQ <- newTBQueue qSize
-  connections <- newTVar M.empty
-  return Server {subscribedQ, connections}
+  subscribers <- newTVar M.empty
+  return Server {subscribedQ, subscribers}
 
 newClient :: Natural -> STM Client
 newClient qSize = do
-  connections <- newTVar M.empty
+  subscriptions <- newTVar M.empty
   rcvQ <- newTBQueue qSize
   sndQ <- newTBQueue qSize
-  return Client {connections, rcvQ, sndQ}
+  return Client {subscriptions, rcvQ, sndQ}
+
+newSubscription :: STM Sub
+newSubscription = do
+  delivered <- newEmptyTMVar
+  return Sub {subThread = NoSub, delivered}
 
 newEnv :: (MonadUnliftIO m, MonadRandom m) => Config -> m Env
 newEnv config = do
diff --git a/src/Server.hs b/src/Server.hs
index 6af048122..862ca6dd4 100644
--- a/src/Server.hs
+++ b/src/Server.hs
@@ -13,6 +13,7 @@
 module Server (runSMPServer) where
 
 import ConnStore
+import Control.Concurrent.STM (stateTVar)
 import Control.Monad
 import Control.Monad.IO.Unlift
 import Control.Monad.Reader
@@ -28,6 +29,7 @@ import Transmission
 import Transport
 import UnliftIO.Async
 import UnliftIO.Concurrent
+import UnliftIO.Exception
 import UnliftIO.IO
 import UnliftIO.STM
 
@@ -42,13 +44,13 @@ runSMPServer cfg@Config {tcpPort} = do
       race_ (runTCPServer tcpPort runClient) (serverThread s)
 
     serverThread :: MonadUnliftIO m => Server -> m ()
-    serverThread Server {subscribedQ, connections} = forever . atomically $ do
+    serverThread Server {subscribedQ, subscribers} = forever . atomically $ do
       (rId, clnt) <- readTBQueue subscribedQ
-      cs <- readTVar connections
+      cs <- readTVar subscribers
       case M.lookup rId cs of
         Just Client {rcvQ} -> writeTBQueue rcvQ (rId, Cmd SBroker END)
         Nothing -> return ()
-      writeTVar connections $ M.insert rId clnt cs
+      writeTVar subscribers $ M.insert rId clnt cs
 
 runClient :: (MonadUnliftIO m, MonadReader Env m) => Handle -> m ()
 runClient h = do
@@ -57,6 +59,17 @@ runClient h = do
   c <- atomically $ newClient q
   s <- asks server
   raceAny_ [send h c, client c s, receive h c]
+    `finally` cancelSubscribers c
+
+cancelSubscribers :: (MonadUnliftIO m) => Client -> m ()
+cancelSubscribers Client {subscriptions} = do
+  cs <- readTVarIO subscriptions
+  forM_ cs cancelSub
+
+cancelSub :: (MonadUnliftIO m) => Sub -> m ()
+cancelSub = \case
+  Sub {subThread = SubThread t} -> killThread t
+  _ -> return ()
 
 raceAny_ :: MonadUnliftIO m => [m a] -> m ()
 raceAny_ = r []
@@ -103,7 +116,7 @@ verifyTransmission signature connId cmd = do
     authErr = smpErr AUTH
 
 client :: forall m. (MonadUnliftIO m, MonadReader Env m) => Client -> Server -> m ()
-client clnt@Client {connections, rcvQ, sndQ} Server {subscribedQ} =
+client clnt@Client {subscriptions, rcvQ, sndQ} Server {subscribedQ} =
   forever $
     atomically (readTBQueue rcvQ)
       >>= processCommand
@@ -113,13 +126,13 @@ client clnt@Client {connections, rcvQ, sndQ} Server {subscribedQ} =
     processCommand (connId, cmd) = do
       st <- asks connStore
       case cmd of
-        Cmd SBroker END -> unsubscribeConn connId >> return (connId, cmd)
+        Cmd SBroker END -> unsubscribeConn >> return (connId, cmd)
         Cmd SBroker _ -> return (connId, cmd)
-        Cmd SSender (SEND msgBody) -> sendMessage st connId msgBody
+        Cmd SSender (SEND msgBody) -> sendMessage st msgBody
         Cmd SRecipient command -> case command of
           CONN rKey -> createConn st rKey
           SUB -> subscribeConn connId
-          ACK -> deliverMessage tryDelPeekMsg connId -- TODO? sending ACK without message loses the message
+          ACK -> acknowledgeMsg
           KEY sKey -> okResponse <$> secureConn st connId sKey
           OFF -> okResponse <$> suspendConn st connId
           DEL -> okResponse <$> deleteConn st connId
@@ -146,24 +159,36 @@ client clnt@Client {connections, rcvQ, sndQ} Server {subscribedQ} =
         subscribeConn :: RecipientId -> m Signed
         subscribeConn rId = do
           atomically $ do
-            cs <- readTVar connections
-            when (M.notMember rId cs) $ do
-              writeTBQueue subscribedQ (rId, clnt)
-              writeTVar connections $ M.insert rId (Left ()) cs
+            cs <- readTVar subscriptions
+            case M.lookup rId cs of
+              Just Sub {delivered} -> void $ tryTakeTMVar delivered
+              Nothing -> do
+                writeTBQueue subscribedQ (rId, clnt)
+                sub <- newSubscription
+                writeTVar subscriptions $ M.insert rId sub cs
           deliverMessage tryPeekMsg rId
 
-        unsubscribeConn :: RecipientId -> m ()
-        unsubscribeConn rId = do
-          cs <- readTVarIO connections
-          atomically . writeTVar connections $ M.delete rId cs
-          case M.lookup rId cs of
-            Just (Right threadId) -> killThread threadId
-            _ -> return ()
+        unsubscribeConn :: m ()
+        unsubscribeConn = do
+          sub <- atomically . stateTVar subscriptions $
+            \cs -> (M.lookup connId cs, M.delete connId cs)
+          mapM_ cancelSub sub
 
-        sendMessage :: MonadConnStore s m => s -> SenderId -> MsgBody -> m Signed
-        sendMessage st sId msgBody =
-          getConn st SSender sId
-            >>= fmap (mkSigned sId) . either (return . ERR) storeMessage
+        acknowledgeMsg :: m Signed
+        acknowledgeMsg = do
+          dlvrd <- atomically $ do
+            cs <- readTVar subscriptions
+            case M.lookup connId cs of
+              Just s -> tryTakeTMVar (delivered s)
+              Nothing -> return Nothing
+          case dlvrd of
+            Just () -> deliverMessage tryDelPeekMsg connId
+            Nothing -> return . mkSigned connId $ ERR PROHIBITED
+
+        sendMessage :: MonadConnStore s m => s -> MsgBody -> m Signed
+        sendMessage st msgBody =
+          getConn st SSender connId
+            >>= fmap (mkSigned connId) . either (return . ERR) storeMessage
           where
             mkMessage :: m Message
             mkMessage = do
@@ -185,24 +210,40 @@ client clnt@Client {connections, rcvQ, sndQ} Server {subscribedQ} =
           ms <- asks msgStore
           q <- getMsgQueue ms rId
           tryPeek q >>= \case
-            Just msg -> return $ msgResponse rId msg
-            Nothing -> forkSubscriber q rId
-
-        forkSubscriber :: MsgQueue -> RecipientId -> m Signed
-        forkSubscriber q rId = do
-          cs <- readTVarIO connections
-          case M.lookup rId cs of
-            Just (Left ()) -> do
-              threadId <- forkIO subscriber
-              trackSubscriber $ Right threadId
-              return ok
-            _ -> return ok
+            Just msg -> do
+              atomically $ do
+                sub <- M.lookup rId <$> readTVar subscriptions
+                forM_ sub $ \Sub {delivered} -> tryPutTMVar delivered ()
+              return $ msgResponse rId msg
+            Nothing -> forkSub q >> return ok
           where
-            trackSubscriber sThrd = atomically . modifyTVar connections $ M.insert rId sThrd
-            subscriber = do
+            forkSub :: MsgQueue -> m ()
+            forkSub q = do
+              sub <- M.lookup rId <$> readTVarIO subscriptions
+              case sub of
+                Just Sub {subThread = NoSub} -> do
+                  atomically . setSub $ \s -> s {subThread = SubPending}
+                  t <- forkIO $ subscriber q
+                  atomically . setSub $ \case
+                    s@Sub {subThread = SubPending} -> s {subThread = SubThread t}
+                    s -> s
+                _ -> return ()
+
+            setSub :: (Sub -> Sub) -> STM ()
+            setSub f = modifyTVar subscriptions $ M.adjust f rId
+
+            subscriber :: MsgQueue -> m ()
+            subscriber q = do
               msg <- peekMsg q
-              atomically . writeTBQueue sndQ $ msgResponse rId msg
-              trackSubscriber $ Left ()
+              atomically $ do
+                writeTBQueue sndQ $ msgResponse rId msg
+                -- setSub (\s -> s {subThread = NoSub})
+                cs <- readTVar subscriptions
+                let sub = M.lookup rId cs
+                forM_ sub $ \s@Sub {delivered} -> do
+                  void $ tryPutTMVar delivered ()
+                  let cs' = M.insert rId s {subThread = NoSub} cs
+                  writeTVar subscriptions cs'
 
         mkSigned :: ConnId -> Command 'Broker -> Signed
         mkSigned cId command = (cId, Cmd SBroker command)
diff --git a/tests/Test.hs b/tests/Test.hs
index e23e3464a..899653cfc 100644
--- a/tests/Test.hs
+++ b/tests/Test.hs
@@ -222,11 +222,16 @@ testSwitchSub =
 
       Resp _ (MSG _ _ msg3) <- tGet fromServer rh2
       (msg3, "test3") #== "delivered to the 2nd TCP connection"
-      Resp _ OK <- sendRecv rh1 ("1234", rId, "ACK")
+
+      Resp _ err <- sendRecv rh1 ("1234", rId, "ACK")
+      (err, ERR PROHIBITED) #== "rejects ACK from the 1st TCP connection"
+
+      Resp _ ok3 <- sendRecv rh2 ("1234", rId, "ACK")
+      (ok3, OK) #== "accepts ACK from the 2nd TCP connection"
 
       timeout 1000 (tGet fromServer rh1) >>= \case
         Nothing -> return ()
-        Just _ -> error "nothing should be delivered to the 1st TCPconnection"
+        Just _ -> error "nothing else is delivered to the 1st TCPconnection"
 
 syntaxTests :: SpecWith ()
 syntaxTests = do