diff --git a/src/NATS.Server/NatsServer.cs b/src/NATS.Server/NatsServer.cs
index 0e7a8e9..83c3f2f 100644
--- a/src/NATS.Server/NatsServer.cs
+++ b/src/NATS.Server/NatsServer.cs
@@ -227,6 +227,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     {
         var subList = sender.Account?.SubList ?? _globalAccount.SubList;
         var result = subList.Match(subject);
+        var delivered = false;
 
         // Deliver to plain subscribers
         foreach (var sub in result.PlainSubs)
@@ -235,6 +236,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                 continue;
 
             DeliverMessage(sub, subject, replyTo, headers, payload);
+            delivered = true;
         }
 
         // Deliver to one member of each queue group (round-robin)
@@ -253,10 +255,18 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                 if (sub.Client != null && (sub.Client != sender || (sender.ClientOpts?.Echo ?? true)))
                 {
                     DeliverMessage(sub, subject, replyTo, headers, payload);
+                    delivered = true;
                     break;
                 }
             }
         }
+
+        // No-responders: if nobody received the message and the publisher
+        // opted in, send back a 503 status HMSG on the reply subject.
+        if (!delivered && replyTo != null && sender.ClientOpts?.NoResponders == true)
+        {
+            SendNoResponders(sender, replyTo);
+        }
     }
 
     private static void DeliverMessage(Subscription sub, string subject, string? replyTo,
@@ -273,6 +283,36 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         client.SendMessage(subject, sub.Sid, replyTo, headers, payload);
     }
 
+    private static void SendNoResponders(NatsClient sender, string replyTo)
+    {
+        // Find the sid for a subscription matching the reply subject
+        var sid = string.Empty;
+        foreach (var sub in sender.Subscriptions.Values)
+        {
+            if (SubjectMatch.MatchLiteral(replyTo, sub.Subject))
+            {
+                sid = sub.Sid;
+                break;
+            }
+        }
+
+        // Build: HMSG {replyTo} {sid} {hdrLen} {hdrLen}\r\n{headers}\r\n
+        var headerBlock = "NATS/1.0 503\r\n\r\n"u8;
+        var hdrLen = headerBlock.Length;
+        var controlLine = Encoding.ASCII.GetBytes($"HMSG {replyTo} {sid} {hdrLen} {hdrLen}\r\n");
+
+        var totalLen = controlLine.Length + hdrLen + NatsProtocol.CrLf.Length;
+        var msg = new byte[totalLen];
+        var offset = 0;
+        controlLine.CopyTo(msg.AsSpan(offset));
+        offset += controlLine.Length;
+        headerBlock.CopyTo(msg.AsSpan(offset));
+        offset += hdrLen;
+        NatsProtocol.CrLf.CopyTo(msg.AsSpan(offset));
+
+        sender.QueueOutbound(msg);
+    }
+
     public Account GetOrCreateAccount(string name)
     {
         return _accounts.GetOrAdd(name, n => new Account(n));