diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 0.5.2
+
+- Fix message delivery logic that blocked delivery of all server messages when server per-queue quota exceeded, making it concurrent per SMP queue, not per server.
+
 # 0.5.1
 
 - Fix server subscription logic bug that was leading to memory leak / resource exhaustion in some edge cases.
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -6,7 +6,9 @@
 📢 **v0.5.1 brings a hotfix to the server's subscription management logic, to apply it log in to your server via SSH and run the following command. If you have store log enabled for your server, information about already established queues will be preserved.** If you're doing a custom installation instead of Linode or DigitalOcean you may have to change the path for binary download.
 
 ```sh
-systemctl stop smp-server && curl -L -o /opt/simplex/bin/smp-server https://github.com/simplex-chat/simplexmq/releases/download/v0.5.1/smp-server-ubuntu-20_04-x86-64 && chmod +x /opt/simplex/bin/smp-server && systemctl start smp-server
+systemctl stop smp-server
+curl -L -o /opt/simplex/bin/smp-server https://github.com/simplex-chat/simplexmq/releases/download/v0.5.1/smp-server-ubuntu-20_04-x86-64
+systemctl start smp-server
 ```
 
 ## Message broker for unidirectional (simplex) queues
diff --git a/simplexmq.cabal b/simplexmq.cabal
--- a/simplexmq.cabal
+++ b/simplexmq.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: ff75c96b1b4f19821ceda01f32e8759e89ad661380f324f1ab9eec3d5c223425
+-- hash: f5e649e0c359792d9528dec6e6cf07af772a521d746e66b8cc2b86d8c604afb2
 
 name:           simplexmq
-version:        0.5.1
+version:        0.5.2
 synopsis:       SimpleXMQ message broker
 description:    This package includes <./docs/Simplex-Messaging-Server.html server>,
                 <./docs/Simplex-Messaging-Client.html client> and
@@ -79,7 +79,7 @@
     , cryptonite >=0.27 && <0.30
     , direct-sqlite ==2.3.*
     , directory ==1.3.*
-    , file-embed ==0.0.14.*
+    , file-embed >=0.0.14.0 && <=0.0.15.0
     , filepath ==1.4.*
     , generic-random >=1.3 && <1.5
     , http-types ==0.12.*
@@ -125,7 +125,7 @@
     , cryptonite >=0.27 && <0.30
     , direct-sqlite ==2.3.*
     , directory ==1.3.*
-    , file-embed ==0.0.14.*
+    , file-embed >=0.0.14.0 && <=0.0.15.0
     , filepath ==1.4.*
     , generic-random >=1.3 && <1.5
     , http-types ==0.12.*
@@ -173,7 +173,7 @@
     , cryptostore ==0.2.*
     , direct-sqlite ==2.3.*
     , directory ==1.3.*
-    , file-embed ==0.0.14.*
+    , file-embed >=0.0.14.0 && <=0.0.15.0
     , filepath ==1.4.*
     , generic-random >=1.3 && <1.5
     , http-types ==0.12.*
@@ -232,7 +232,7 @@
     , cryptonite >=0.27 && <0.30
     , direct-sqlite ==2.3.*
     , directory ==1.3.*
-    , file-embed ==0.0.14.*
+    , file-embed >=0.0.14.0 && <=0.0.15.0
     , filepath ==1.4.*
     , generic-random >=1.3 && <1.5
     , hspec ==2.7.*
diff --git a/src/Simplex/Messaging/Agent.hs b/src/Simplex/Messaging/Agent.hs
--- a/src/Simplex/Messaging/Agent.hs
+++ b/src/Simplex/Messaging/Agent.hs
@@ -69,7 +69,6 @@
 import Data.Functor (($>))
 import Data.List.NonEmpty (NonEmpty (..))
 import qualified Data.List.NonEmpty as L
-import Data.Map.Strict (Map)
 import qualified Data.Map.Strict as M
 import Data.Maybe (isJust)
 import qualified Data.Text as T
@@ -87,9 +86,9 @@
 import Simplex.Messaging.Protocol (MsgBody, SenderPublicKey)
 import qualified Simplex.Messaging.Protocol as SMP
 import Simplex.Messaging.Transport (ATransport (..), TProxy, Transport (..), currentSMPVersionStr, runTransportServer)
-import Simplex.Messaging.Util (bshow, tryError)
+import Simplex.Messaging.Util (bshow, tryError, unlessM)
 import System.Random (randomR)
-import UnliftIO.Async (Async, async, race_)
+import UnliftIO.Async (async, race_)
 import qualified UnliftIO.Exception as E
 import UnliftIO.STM
 
@@ -334,7 +333,7 @@
 subscribeConnection' c connId =
   withStore (`getConn` connId) >>= \case
     SomeConn _ (DuplexConnection _ rq sq) -> do
-      resumeDelivery sq
+      resumeMsgDelivery c connId sq
       case status (sq :: SndQueue) of
         Confirmed -> withVerifyKey sq $ \verifyKey -> do
           conf <- withStore (`getAcceptedConfirmation` connId)
@@ -345,7 +344,7 @@
         Active -> subscribeQueue c rq connId
         _ -> throwError $ INTERNAL "unexpected queue status"
     SomeConn _ (SndConnection _ sq) -> do
-      resumeDelivery sq
+      resumeMsgDelivery c connId sq
       case status (sq :: SndQueue) of
         Confirmed -> withVerifyKey sq $ \verifyKey ->
           activateQueueJoining c connId sq verifyKey =<< resumeInterval
@@ -354,12 +353,6 @@
     SomeConn _ (RcvConnection _ rq) -> subscribeQueue c rq connId
     SomeConn _ (ContactConnection _ rq) -> subscribeQueue c rq connId
   where
-    resumeDelivery :: SndQueue -> m ()
-    resumeDelivery SndQueue {server} = do
-      wasDelivering <- resumeMsgDelivery c connId server
-      unless wasDelivering $ do
-        pending <- withStore (`getPendingMsgs` connId)
-        queuePendingMsgs c connId pending
     withVerifyKey :: SndQueue -> (C.PublicKey -> m ()) -> m ()
     withVerifyKey sq action =
       let err = throwError $ INTERNAL "missing signing key public counterpart"
@@ -382,14 +375,10 @@
     _ -> throwError $ CONN SIMPLEX
   where
     enqueueMessage :: SndQueue -> m AgentMsgId
-    enqueueMessage SndQueue {server} = do
+    enqueueMessage sq = do
+      resumeMsgDelivery c connId sq
       msgId <- storeSentMsg
-      wasDelivering <- resumeMsgDelivery c connId server
-      pending <-
-        if wasDelivering
-          then pure [PendingMsg {connId, msgId}]
-          else withStore (`getPendingMsgs` connId)
-      queuePendingMsgs c connId pending
+      queuePendingMsgs c connId sq [msgId]
       pure $ unId msgId
       where
         storeSentMsg :: m InternalId
@@ -410,63 +399,63 @@
             createSndMsg st connId msgData
             pure internalId
 
-resumeMsgDelivery :: forall m. AgentMonad m => AgentClient -> ConnId -> SMPServer -> m Bool
-resumeMsgDelivery c connId srv = do
-  void $ resume srv (srvMsgDeliveries c) $ runSrvMsgDelivery c srv
-  resume connId (connMsgDeliveries c) $ runMsgDelivery c connId srv
+resumeMsgDelivery :: forall m. AgentMonad m => AgentClient -> ConnId -> SndQueue -> m ()
+resumeMsgDelivery c connId sq@SndQueue {server, sndId} = do
+  let qKey = (connId, server, sndId)
+  unlessM (queueDelivering qKey) $
+    async (runSmpQueueMsgDelivery c connId sq)
+      >>= atomically . modifyTVar (smpQueueMsgDeliveries c) . M.insert qKey
+  unlessM connQueued $
+    withStore (`getPendingMsgs` connId)
+      >>= queuePendingMsgs c connId sq
   where
-    resume :: Ord a => a -> TVar (Map a (Async ())) -> m () -> m Bool
-    resume key actionMap actionProcess = do
-      isDelivering <- isJust . M.lookup key <$> readTVarIO actionMap
-      unless isDelivering $
-        async actionProcess
-          >>= atomically . modifyTVar actionMap . M.insert key
-      pure isDelivering
+    queueDelivering qKey = isJust . M.lookup qKey <$> readTVarIO (smpQueueMsgDeliveries c)
+    connQueued =
+      atomically $
+        isJust
+          <$> stateTVar
+            (connMsgsQueued c)
+            (\m -> (M.lookup connId m, M.insert connId True m))
 
-queuePendingMsgs :: AgentMonad m => AgentClient -> ConnId -> [PendingMsg] -> m ()
-queuePendingMsgs c connId pending =
-  atomically $ getPendingMsgQ connId (connMsgQueues c) >>= forM_ pending . writeTQueue
+queuePendingMsgs :: AgentMonad m => AgentClient -> ConnId -> SndQueue -> [InternalId] -> m ()
+queuePendingMsgs c connId sq msgIds = atomically $ do
+  q <- getPendingMsgQ c connId sq
+  mapM_ (writeTQueue q) msgIds
 
-getPendingMsgQ :: Ord a => a -> TVar (Map a (TQueue PendingMsg)) -> STM (TQueue PendingMsg)
-getPendingMsgQ key queueMap = do
-  maybe newMsgQueue pure . M.lookup key =<< readTVar queueMap
+getPendingMsgQ :: AgentClient -> ConnId -> SndQueue -> STM (TQueue InternalId)
+getPendingMsgQ c connId SndQueue {server, sndId} = do
+  let qKey = (connId, server, sndId)
+  maybe (newMsgQueue qKey) pure . M.lookup qKey =<< readTVar (smpQueueMsgQueues c)
   where
-    newMsgQueue :: STM (TQueue PendingMsg)
-    newMsgQueue = do
+    newMsgQueue qKey = do
       mq <- newTQueue
-      modifyTVar queueMap $ M.insert key mq
+      modifyTVar (smpQueueMsgQueues c) $ M.insert qKey mq
       pure mq
 
-runMsgDelivery :: AgentMonad m => AgentClient -> ConnId -> SMPServer -> m ()
-runMsgDelivery c connId srv = do
-  mq <- atomically . getPendingMsgQ connId $ connMsgQueues c
-  smq <- atomically . getPendingMsgQ srv $ srvMsgQueues c
-  forever . atomically $ readTQueue mq >>= writeTQueue smq
-
-runSrvMsgDelivery :: forall m. AgentMonad m => AgentClient -> SMPServer -> m ()
-runSrvMsgDelivery c@AgentClient {subQ} srv = do
-  mq <- atomically . getPendingMsgQ srv $ srvMsgQueues c
+runSmpQueueMsgDelivery :: forall m. AgentMonad m => AgentClient -> ConnId -> SndQueue -> m ()
+runSmpQueueMsgDelivery c@AgentClient {subQ} connId sq = do
+  mq <- atomically $ getPendingMsgQ c connId sq
   ri <- asks $ reconnectInterval . config
   forever $ do
-    PendingMsg {connId, msgId} <- atomically $ readTQueue mq
+    msgId <- atomically $ readTQueue mq
     let mId = unId msgId
     withStore (\st -> E.try $ getPendingMsgData st connId msgId) >>= \case
       Left (e :: E.SomeException) ->
-        notify connId $ MERR mId (INTERNAL $ show e)
-      Right (sq, msgBody) -> do
+        notify $ MERR mId (INTERNAL $ show e)
+      Right msgBody -> do
         withRetryInterval ri $ \loop -> do
           tryError (sendAgentMessage c sq msgBody) >>= \case
             Left e -> case e of
               SMP SMP.QUOTA -> loop
-              SMP {} -> notify connId $ MERR mId e
-              CMD {} -> notify connId $ MERR mId e
+              SMP {} -> notify $ MERR mId e
+              CMD {} -> notify $ MERR mId e
               _ -> loop
             Right () -> do
-              notify connId $ SENT mId
+              notify $ SENT mId
               withStore $ \st -> updateSndMsgStatus st connId msgId SndMsgSent
   where
-    notify :: ConnId -> ACommand 'Agent -> m ()
-    notify connId cmd = atomically $ writeTBQueue subQ ("", connId, cmd)
+    notify :: ACommand 'Agent -> m ()
+    notify cmd = atomically $ writeTBQueue subQ ("", connId, cmd)
 
 ackMessage' :: forall m. AgentMonad m => AgentClient -> ConnId -> AgentMsgId -> m ()
 ackMessage' c connId msgId = do
diff --git a/src/Simplex/Messaging/Agent/Client.hs b/src/Simplex/Messaging/Agent/Client.hs
--- a/src/Simplex/Messaging/Agent/Client.hs
+++ b/src/Simplex/Messaging/Agent/Client.hs
@@ -60,6 +60,7 @@
 import Simplex.Messaging.Client
 import qualified Simplex.Messaging.Crypto as C
 import Simplex.Messaging.Protocol (ErrorType (AUTH), MsgBody, QueueId, SenderPublicKey)
+import qualified Simplex.Messaging.Protocol as SMP
 import Simplex.Messaging.Util (bshow, liftEitherError, liftError)
 import UnliftIO.Exception (IOException)
 import qualified UnliftIO.Exception as E
@@ -73,10 +74,9 @@
     subscrSrvrs :: TVar (Map SMPServer (Map ConnId RcvQueue)),
     subscrConns :: TVar (Map ConnId SMPServer),
     activations :: TVar (Map ConnId (Async ())), -- activations of send queues in progress
-    connMsgQueues :: TVar (Map ConnId (TQueue PendingMsg)),
-    connMsgDeliveries :: TVar (Map ConnId (Async ())),
-    srvMsgQueues :: TVar (Map SMPServer (TQueue PendingMsg)),
-    srvMsgDeliveries :: TVar (Map SMPServer (Async ())),
+    connMsgsQueued :: TVar (Map ConnId Bool),
+    smpQueueMsgQueues :: TVar (Map (ConnId, SMPServer, SMP.SenderId) (TQueue InternalId)),
+    smpQueueMsgDeliveries :: TVar (Map (ConnId, SMPServer, SMP.SenderId) (Async ())),
     reconnections :: TVar [Async ()],
     clientId :: Int,
     agentEnv :: Env,
@@ -94,14 +94,13 @@
   subscrSrvrs <- newTVar M.empty
   subscrConns <- newTVar M.empty
   activations <- newTVar M.empty
-  connMsgQueues <- newTVar M.empty
-  connMsgDeliveries <- newTVar M.empty
-  srvMsgQueues <- newTVar M.empty
-  srvMsgDeliveries <- newTVar M.empty
+  connMsgsQueued <- newTVar M.empty
+  smpQueueMsgQueues <- newTVar M.empty
+  smpQueueMsgDeliveries <- newTVar M.empty
   reconnections <- newTVar []
   clientId <- stateTVar (clientCounter agentEnv) $ \i -> (i + 1, i + 1)
   lock <- newTMVar ()
-  return AgentClient {rcvQ, subQ, msgQ, smpClients, subscrSrvrs, subscrConns, activations, connMsgQueues, connMsgDeliveries, srvMsgQueues, srvMsgDeliveries, reconnections, clientId, agentEnv, smpSubscriber = undefined, lock}
+  return AgentClient {rcvQ, subQ, msgQ, smpClients, subscrSrvrs, subscrConns, activations, connMsgsQueued, smpQueueMsgQueues, smpQueueMsgDeliveries, reconnections, clientId, agentEnv, smpSubscriber = undefined, lock}
 
 -- | Agent monad with MonadReader Env and MonadError AgentErrorType
 type AgentMonad m = (MonadUnliftIO m, MonadReader Env m, MonadError AgentErrorType m)
@@ -177,8 +176,7 @@
   closeSMPServerClients c
   cancelActions $ activations c
   cancelActions $ reconnections c
-  cancelActions $ connMsgDeliveries c
-  cancelActions $ srvMsgDeliveries c
+  cancelActions $ smpQueueMsgDeliveries c
 
 closeSMPServerClients :: AgentClient -> IO ()
 closeSMPServerClients c = readTVarIO (smpClients c) >>= mapM_ closeSMPClient
diff --git a/src/Simplex/Messaging/Agent/Store.hs b/src/Simplex/Messaging/Agent/Store.hs
--- a/src/Simplex/Messaging/Agent/Store.hs
+++ b/src/Simplex/Messaging/Agent/Store.hs
@@ -63,8 +63,8 @@
   updateSndIds :: s -> ConnId -> m (InternalId, InternalSndId, PrevSndMsgHash)
   createSndMsg :: s -> ConnId -> SndMsgData -> m ()
   updateSndMsgStatus :: s -> ConnId -> InternalId -> SndMsgStatus -> m ()
-  getPendingMsgData :: s -> ConnId -> InternalId -> m (SndQueue, MsgBody)
-  getPendingMsgs :: s -> ConnId -> m [PendingMsg]
+  getPendingMsgData :: s -> ConnId -> InternalId -> m MsgBody
+  getPendingMsgs :: s -> ConnId -> m [InternalId]
   getMsg :: s -> ConnId -> InternalId -> m Msg
   checkRcvMsg :: s -> ConnId -> InternalId -> m ()
   updateRcvMsgAck :: s -> ConnId -> InternalId -> m ()
diff --git a/src/Simplex/Messaging/Agent/Store/SQLite.hs b/src/Simplex/Messaging/Agent/Store/SQLite.hs
--- a/src/Simplex/Messaging/Agent/Store/SQLite.hs
+++ b/src/Simplex/Messaging/Agent/Store/SQLite.hs
@@ -461,34 +461,28 @@
           ":snd_status" := msgStatus
         ]
 
-  getPendingMsgData :: SQLiteStore -> ConnId -> InternalId -> m (SndQueue, MsgBody)
+  getPendingMsgData :: SQLiteStore -> ConnId -> InternalId -> m MsgBody
   getPendingMsgData st connId msgId =
-    liftIOEither . withTransaction st $ \db -> runExceptT $ do
-      sq <- ExceptT $ sndQueue <$> getSndQueueByConnAlias_ db connId
-      msgBody <-
-        ExceptT $
-          sndMsgData
-            <$> DB.query
-              db
-              [sql|
-                SELECT m.msg_body
-                FROM messages m
-                JOIN snd_messages s ON s.conn_alias = m.conn_alias AND s.internal_id = m.internal_id
-                WHERE m.conn_alias = ? AND m.internal_id = ?
-              |]
-              (connId, msgId)
-      pure (sq, msgBody)
+    liftIOEither . withTransaction st $ \db ->
+      sndMsgData
+        <$> DB.query
+          db
+          [sql|
+            SELECT m.msg_body
+            FROM messages m
+            JOIN snd_messages s ON s.conn_alias = m.conn_alias AND s.internal_id = m.internal_id
+            WHERE m.conn_alias = ? AND m.internal_id = ?
+          |]
+          (connId, msgId)
     where
       sndMsgData :: [Only MsgBody] -> Either StoreError MsgBody
       sndMsgData [Only msgBody] = Right msgBody
       sndMsgData _ = Left SEMsgNotFound
-      sndQueue :: Maybe SndQueue -> Either StoreError SndQueue
-      sndQueue = maybe (Left SEConnNotFound) Right
 
-  getPendingMsgs :: SQLiteStore -> ConnId -> m [PendingMsg]
+  getPendingMsgs :: SQLiteStore -> ConnId -> m [InternalId]
   getPendingMsgs st connId =
     liftIO . withTransaction st $ \db ->
-      map (PendingMsg connId . fromOnly)
+      map fromOnly
         <$> DB.query db "SELECT internal_id FROM snd_messages WHERE conn_alias = ? AND snd_status = ?" (connId, SndMsgCreated)
 
   getMsg :: SQLiteStore -> ConnId -> InternalId -> m Msg
diff --git a/src/Simplex/Messaging/Util.hs b/src/Simplex/Messaging/Util.hs
--- a/src/Simplex/Messaging/Util.hs
+++ b/src/Simplex/Messaging/Util.hs
@@ -53,3 +53,11 @@
 
 tryError :: MonadError e m => m a -> m (Either e a)
 tryError action = (Right <$> action) `catchError` (pure . Left)
+
+ifM :: Monad m => m Bool -> m a -> m a -> m a
+ifM ba t f = ba >>= \b -> if b then t else f
+{-# INLINE ifM #-}
+
+unlessM :: Monad m => m Bool -> m () -> m ()
+unlessM b = ifM b $ pure ()
+{-# INLINE unlessM #-}
diff --git a/tests/AgentTests.hs b/tests/AgentTests.hs
--- a/tests/AgentTests.hs
+++ b/tests/AgentTests.hs
@@ -14,6 +14,7 @@
 import AgentTests.FunctionalAPITests (functionalAPITests)
 import AgentTests.SQLiteTests (storeTests)
 import Control.Concurrent
+import Control.Monad (forM_)
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
 import Network.HTTP.Types (urlEncode)
@@ -23,6 +24,7 @@
 import qualified Simplex.Messaging.Agent.Protocol as A
 import Simplex.Messaging.Protocol (ErrorType (..), MsgBody)
 import Simplex.Messaging.Transport (ATransport (..), TProxy (..), Transport (..))
+import Simplex.Messaging.Util (bshow)
 import System.Directory (removeFile)
 import System.Timeout
 import Test.Hspec
@@ -63,6 +65,10 @@
       smpAgentTest2_2_2_needs_server $ testMsgDeliveryServerRestart t
     it "should deliver pending messages after agent restarting" $
       smpAgentTest1_1_1 $ testMsgDeliveryAgentRestart t
+    it "should concurrently deliver messages to connections without blocking" $
+      smpAgentTest2_2_1 $ testConcurrentMsgDelivery t
+    it "should deliver messages if one of connections has quota exceeded" $
+      smpAgentTest2_2_1 $ testMsgDeliveryQuotaExceeded t
 
 -- | receive message to handle `h`
 (<#:) :: Transport c => c -> IO (ATransmissionOrError 'Agent)
@@ -325,6 +331,47 @@
     withServer test' = withSmpServerStoreLogOn (ATransport t) testPort2 (const test') `shouldReturn` ()
     withAgent = withSmpAgentThreadOn_ (ATransport t) (agentTestPort, testPort, testDB) (pure ()) . const . testSMPAgentClientOn agentTestPort
 
+testConcurrentMsgDelivery :: Transport c => TProxy c -> c -> c -> IO ()
+testConcurrentMsgDelivery _ alice bob = do
+  connect (alice, "alice") (bob, "bob")
+
+  ("1", "bob2", Right (INV cReq)) <- alice #: ("1", "bob2", "NEW INV")
+  let cReq' = serializeConnReq cReq
+  bob #: ("11", "alice2", "JOIN " <> cReq' <> " 14\nbob's connInfo") #> ("11", "alice2", OK)
+  ("", "bob2", Right (CONF _confId "bob's connInfo")) <- (alice <#:)
+  -- below commands would be needed to accept bob's connection, but alice does not
+  -- alice #: ("2", "bob", "LET " <> _confId <> " 16\nalice's connInfo") #> ("2", "bob", OK)
+  -- bob <# ("", "alice", INFO "alice's connInfo")
+  -- bob <# ("", "alice", CON)
+  -- alice <# ("", "bob", CON)
+
+  -- the first connection should not be blocked by the second one
+  sendMessage (alice, "alice") (bob, "bob") "hello"
+  -- alice #: ("2", "bob", "SEND :hello") #> ("2", "bob", MID 1)
+  -- alice <# ("", "bob", SENT 1)
+  -- bob <#= \case ("", "alice", Msg "hello") -> True; _ -> False
+  -- bob #: ("12", "alice", "ACK 1") #> ("12", "alice", OK)
+  bob #: ("14", "alice", "SEND 9\nhello too") #> ("14", "alice", MID 2)
+  bob <# ("", "alice", SENT 2)
+  -- if delivery is blocked it won't go further
+  alice <#= \case ("", "bob", Msg "hello too") -> True; _ -> False
+  alice #: ("3", "bob", "ACK 2") #> ("3", "bob", OK)
+
+testMsgDeliveryQuotaExceeded :: Transport c => TProxy c -> c -> c -> IO ()
+testMsgDeliveryQuotaExceeded _ alice bob = do
+  connect (alice, "alice") (bob, "bob")
+  connect (alice, "alice2") (bob, "bob2")
+  forM_ [1 .. 4 :: Int] $ \i -> do
+    let corrId = bshow i
+        msg = "message " <> bshow i
+    (_, "bob", Right (MID mId)) <- alice #: (corrId, "bob", "SEND :" <> msg)
+    alice <#= \case ("", "bob", SENT m) -> m == mId; _ -> False
+  (_, "bob", Right (MID _)) <- alice #: ("5", "bob", "SEND :over quota")
+
+  alice #: ("1", "bob2", "SEND :hello") #> ("1", "bob2", MID 1)
+  -- if delivery is blocked it won't go further
+  alice <# ("", "bob2", SENT 1)
+
 connect :: forall c. Transport c => (c, ByteString) -> (c, ByteString) -> IO ()
 connect (h1, name1) (h2, name2) = do
   ("c1", _, Right (INV cReq)) <- h1 #: ("c1", name2, "NEW INV")
@@ -335,6 +382,16 @@
   h2 <# ("", name1, INFO "info1")
   h2 <# ("", name1, CON)
   h1 <# ("", name2, CON)
+
+sendMessage :: Transport c => (c, ConnId) -> (c, ConnId) -> ByteString -> IO ()
+sendMessage (h1, name1) (h2, name2) msg = do
+  ("m1", name2', Right (MID mId)) <- h1 #: ("m1", name2, "SEND :" <> msg)
+  name2' `shouldBe` name2
+  h1 <#= \case ("", n, SENT m) -> n == name2 && m == mId; _ -> False
+  ("", name1', Right (MSG MsgMeta {recipient = (msgId, _)} msg')) <- (h2 <#:)
+  name1' `shouldBe` name1
+  msg' `shouldBe` msg
+  h2 #: ("m2", name1, "ACK " <> bshow msgId) =#> \case ("m2", n, OK) -> n == name1; _ -> False
 
 -- connect' :: forall c. Transport c => c -> c -> IO (ByteString, ByteString)
 -- connect' h1 h2 = do