diff --git a/Build/MQTTnet.nuspec b/Build/MQTTnet.nuspec
index 16e34e78..6dd2330a 100644
--- a/Build/MQTTnet.nuspec
+++ b/Build/MQTTnet.nuspec
@@ -17,7 +17,7 @@
 * [Server] Adjusted some namespaces (BREAKING CHANGE!)
 * [Server] Added state checks (throw if not started etc.) for most server APIs.
 * [Server] Exposed real X509Certificate2 (instead byte array) to TLS options (thanks to @borigas).
-* [Server] Fixed memory leak with TCP sockets (MqttServer is now sealed!) (BREAKING CHANGE!)
+* [Server] Fixed memory leak with TCP sockets (MqttServer is now Disposable!) (BREAKING CHANGE!)
 * [Core] Fixed a null reference exception in the MqttTcpChannel with WriteAsync and ReadAsync.
 * [Core] Added server interceptor for undelivered messages (thanks to @cshark-inator).
 * [nuget] Added support for SourceLink (thanks to @JTOne123).
diff --git a/Source/MQTTnet/Adapter/MqttChannelAdapter.cs b/Source/MQTTnet/Adapter/MqttChannelAdapter.cs
index f991277b..2efcbaee 100644
--- a/Source/MQTTnet/Adapter/MqttChannelAdapter.cs
+++ b/Source/MQTTnet/Adapter/MqttChannelAdapter.cs
@@ -16,8 +16,8 @@ namespace MQTTnet.Adapter
 {
     public sealed class MqttChannelAdapter : Disposable, IMqttChannelAdapter
     {
-        const uint _errorOperationAborted = 0x800703E3;
-        const int _readBufferSize = 4096;  // TODO: Move buffer size to config
+        const uint ErrorOperationAborted = 0x800703E3;
+        const int ReadBufferSize = 4096;
 
         readonly IMqttNetScopedLogger _logger;
         readonly IMqttChannel _channel;
@@ -25,7 +25,7 @@ namespace MQTTnet.Adapter
 
         readonly byte[] _fixedHeaderBuffer = new byte[2];
 
-        readonly SemaphoreSlim _writerSemaphore = new SemaphoreSlim(1, 1);
+        readonly AsyncLock _syncRoot = new AsyncLock();
 
         long _bytesReceived;
         long _bytesSent;
@@ -60,7 +60,7 @@ namespace MQTTnet.Adapter
         {
             cancellationToken.ThrowIfCancellationRequested();
             ThrowIfDisposed();
-            
+
             try
             {
                 if (timeout == TimeSpan.Zero)
@@ -87,7 +87,7 @@ namespace MQTTnet.Adapter
         {
             cancellationToken.ThrowIfCancellationRequested();
             ThrowIfDisposed();
-            
+
             try
             {
                 if (timeout == TimeSpan.Zero)
@@ -115,56 +115,51 @@ namespace MQTTnet.Adapter
         {
             cancellationToken.ThrowIfCancellationRequested();
             ThrowIfDisposed();
-            
+
             try
             {
-                await _writerSemaphore.WaitAsync(cancellationToken).ConfigureAwait(false);
+                using (await _syncRoot.WaitAsync(cancellationToken).ConfigureAwait(false))
+                {
+                    // Check for cancellation here again because "WaitAsync" might take some time.
+                    cancellationToken.ThrowIfCancellationRequested();
+
+                    try
+                    {
+                        var packetData = PacketFormatterAdapter.Encode(packet);
+
+                        if (timeout == TimeSpan.Zero)
+                        {
+                            await _channel.WriteAsync(packetData.Array, packetData.Offset, packetData.Count, cancellationToken).ConfigureAwait(false);
+                        }
+                        else
+                        {
+                            await MqttTaskTimeout.WaitAsync(
+                                t => _channel.WriteAsync(packetData.Array, packetData.Offset, packetData.Count, t), timeout, cancellationToken).ConfigureAwait(false);
+                        }
+
+                        Interlocked.Add(ref _bytesReceived, packetData.Count);
+
+                        _logger.Verbose("TX ({0} bytes) >>> {1}", packetData.Count, packet);
+                    }
+                    catch (Exception exception)
+                    {
+                        if (IsWrappedException(exception))
+                        {
+                            throw;
+                        }
+
+                        WrapException(exception);
+                    }
+                    finally
+                    {
+                        PacketFormatterAdapter.FreeBuffer();
+                    }
+                }
             }
             catch (ObjectDisposedException)
             {
                 throw new OperationCanceledException();
             }
-
-            try
-            {
-                var packetData = PacketFormatterAdapter.Encode(packet);
-
-                if (timeout == TimeSpan.Zero)
-                {
-                    await _channel.WriteAsync(packetData.Array, packetData.Offset, packetData.Count, cancellationToken).ConfigureAwait(false);
-                }
-                else
-                {
-                    await MqttTaskTimeout.WaitAsync(
-                        t => _channel.WriteAsync(packetData.Array, packetData.Offset, packetData.Count, t), timeout, cancellationToken).ConfigureAwait(false);
-                }
-
-                Interlocked.Add(ref _bytesReceived, packetData.Count);
-
-                _logger.Verbose("TX ({0} bytes) >>> {1}", packetData.Count, packet);
-            }
-            catch (Exception exception)
-            {
-                if (IsWrappedException(exception))
-                {
-                    throw;
-                }
-
-                WrapException(exception);
-            }
-            finally
-            {
-                PacketFormatterAdapter.FreeBuffer();
-
-                try
-                {
-                    _writerSemaphore.Release();
-                }
-                catch (ObjectDisposedException)
-                {
-                    throw new OperationCanceledException();
-                }
-            }
         }
 
         public async Task<MqttBasePacket> ReceivePacketAsync(TimeSpan timeout, CancellationToken cancellationToken)
@@ -236,7 +231,7 @@ namespace MQTTnet.Adapter
             if (disposing)
             {
                 _channel.Dispose();
-                _writerSemaphore.Dispose();
+                _syncRoot.Dispose();
             }
 
             base.Dispose(disposing);
@@ -268,7 +263,7 @@ namespace MQTTnet.Adapter
 
                 var body = new byte[fixedHeader.RemainingLength];
                 var bodyOffset = 0;
-                var chunkSize = Math.Min(_readBufferSize, fixedHeader.RemainingLength);
+                var chunkSize = Math.Min(ReadBufferSize, fixedHeader.RemainingLength);
 
                 do
                 {
@@ -327,7 +322,7 @@ namespace MQTTnet.Adapter
 
             if (exception is COMException comException)
             {
-                if ((uint)comException.HResult == _errorOperationAborted)
+                if ((uint)comException.HResult == ErrorOperationAborted)
                 {
                     throw new OperationCanceledException();
                 }
diff --git a/Tests/MQTTnet.Core.Tests/MqttClient_Tests.cs b/Tests/MQTTnet.Core.Tests/MqttClient_Tests.cs
index 4d11da39..87c98c98 100644
--- a/Tests/MQTTnet.Core.Tests/MqttClient_Tests.cs
+++ b/Tests/MQTTnet.Core.Tests/MqttClient_Tests.cs
@@ -125,6 +125,8 @@ namespace MQTTnet.Tests
 
                 await client1.PublishAsync("request", null, MqttQualityOfServiceLevel.AtLeastOnce);
 
+                await Task.Delay(500);
+
                 SpinWait.SpinUntil(() => replyReceived, TimeSpan.FromSeconds(10));
 
                 Assert.IsTrue(replyReceived);
diff --git a/Tests/MQTTnet.Core.Tests/Server_Tests.cs b/Tests/MQTTnet.Core.Tests/Server_Tests.cs
index 2db3f614..244c1223 100644
--- a/Tests/MQTTnet.Core.Tests/Server_Tests.cs
+++ b/Tests/MQTTnet.Core.Tests/Server_Tests.cs
@@ -304,7 +304,7 @@ namespace MQTTnet.Tests
                     await c2.PublishAsync(messageBuilder.Build()).ConfigureAwait(false);
                 }
 
-                SpinWait.SpinUntil(() => receivedMessagesCount == 500, 2000);
+                SpinWait.SpinUntil(() => receivedMessagesCount == 500, 5000);
 
                 Assert.AreEqual(500, receivedMessagesCount);
             }
@@ -325,7 +325,7 @@ namespace MQTTnet.Tests
                 for (var i = 0; i < 500; i++)
                 {
                     var so = new MqttClientSubscribeOptionsBuilder()
-                        .WithTopicFilter(i.ToString(), MqttQualityOfServiceLevel.AtMostOnce).Build();
+                        .WithTopicFilter(i.ToString()).Build();
 
                     await c1.SubscribeAsync(so).ConfigureAwait(false);
                 }
@@ -340,7 +340,7 @@ namespace MQTTnet.Tests
                     await c2.PublishAsync(messageBuilder.Build()).ConfigureAwait(false);
                 }
 
-                SpinWait.SpinUntil(() => receivedMessagesCount == 500, 2000);
+                SpinWait.SpinUntil(() => receivedMessagesCount == 500, 5000);
 
                 Assert.AreEqual(500, receivedMessagesCount);
             }