diff --git a/scuttlebutt/src/digest.rs b/scuttlebutt/src/digest.rs
index 8975992..c672b47 100644
--- a/scuttlebutt/src/digest.rs
+++ b/scuttlebutt/src/digest.rs
@@ -27,7 +27,7 @@ use crate::{NodeId, Version};
 ///
 /// It is equivalent to a map
 /// peer -> max version.
-#[derive(Debug, Default)]
+#[derive(Debug, Default, PartialEq)]
 pub struct Digest {
     pub(crate) node_max_version: BTreeMap<NodeId, Version>,
 }
diff --git a/scuttlebutt/src/lib.rs b/scuttlebutt/src/lib.rs
index 6b8aae7..f08e60a 100644
--- a/scuttlebutt/src/lib.rs
+++ b/scuttlebutt/src/lib.rs
@@ -187,9 +187,9 @@ impl ScuttleButt {
                 if cluster_name != self.cluster_name {
                     warn!(
                         cluster_name = ?cluster_name,
-                        "ignoring syn message with mismatching cluster name"
+                        "rejecting syn message with mismatching cluster name"
                     );
-                    return None;
+                    return Some(ScuttleButtMessage::BadCluster);
                 }
 
                 let self_digest = self.compute_digest();
@@ -219,6 +219,10 @@ impl ScuttleButt {
                 self.cluster_state.apply_delta(delta);
                 None
             }
+            ScuttleButtMessage::BadCluster => {
+                warn!("message rejected by peer: cluster name mismatch");
+                return None;
+            }
         }
     }
 
diff --git a/scuttlebutt/src/message.rs b/scuttlebutt/src/message.rs
index e4504f4..bf3a35f 100644
--- a/scuttlebutt/src/message.rs
+++ b/scuttlebutt/src/message.rs
@@ -31,7 +31,7 @@ use crate::serialize::Serializable;
 /// between node A and node B.
 /// The names {Syn, SynAck, Ack} of the different steps are borrowed from
 /// TCP Handshake.
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum ScuttleButtMessage {
     /// Node A initiates handshakes.
     Syn {
@@ -44,6 +44,9 @@ pub enum ScuttleButtMessage {
     SynAck { digest: Digest, delta: Delta },
     /// Node A returns a partial update for B.
     Ack { delta: Delta },
+    /// Node B rejects the Syn message because of a
+    /// cluster name mismatch between the peers.
+    BadCluster,
 }
 
 #[derive(Copy, Clone)]
@@ -52,6 +55,7 @@ enum MessageType {
     Syn = 0,
     SynAck = 1u8,
     Ack = 2u8,
+    BadCluster = 3u8,
 }
 
 impl MessageType {
@@ -60,6 +64,7 @@ impl MessageType {
             0 => Some(Self::Syn),
             1 => Some(Self::SynAck),
             2 => Some(Self::Ack),
+            3 => Some(Self::BadCluster),
             _ => None,
         }
     }
@@ -88,6 +93,9 @@ impl Serializable for ScuttleButtMessage {
                 buf.push(MessageType::Ack.to_code());
                 delta.serialize(buf);
             }
+            ScuttleButtMessage::BadCluster => {
+                buf.push(MessageType::BadCluster.to_code());
+            }
         }
     }
 
@@ -116,6 +124,7 @@ impl Serializable for ScuttleButtMessage {
                 let delta = Delta::deserialize(buf)?;
                 Ok(Self::Ack { delta })
             }
+            MessageType::BadCluster => Ok(Self::BadCluster),
         }
     }
 
@@ -129,6 +138,30 @@ impl Serializable for ScuttleButtMessage {
                 1 + digest.serialized_len() + delta.serialized_len()
             }
             ScuttleButtMessage::Ack { delta } => 1 + delta.serialized_len(),
+            ScuttleButtMessage::BadCluster => 1,
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::{Digest, ScuttleButtMessage};
+    use crate::serialize::test_serdeser_aux;
+
+    #[test]
+    fn test_syn() {
+        let mut digest = Digest::default();
+        digest.add_node("node1".into(), 1);
+        digest.add_node("node2".into(), 2);
+        let syn = ScuttleButtMessage::Syn {
+            cluster_name: "cluster-a".to_string(),
+            digest
+        };
+        test_serdeser_aux(&syn, 58);
+    }
+
+    #[test]
+    fn test_bad_cluster() {
+        test_serdeser_aux(&ScuttleButtMessage::BadCluster, 1);
+    }
+}
diff --git a/scuttlebutt/src/server.rs b/scuttlebutt/src/server.rs
index a29c188..94903d8 100644
--- a/scuttlebutt/src/server.rs
+++ b/scuttlebutt/src/server.rs
@@ -463,18 +463,19 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn ignore_mismatched_cluster_name() {
-        let server_addr = "0.0.0.0:2223";
-        let socket = UdpSocket::bind("0.0.0.0:2224").await.unwrap();
+    async fn syn_bad_cluster() {
+        let outsider_addr = "0.0.0.0:2224";
+        let socket = UdpSocket::bind(outsider_addr).await.unwrap();
         let mut outsider = ScuttleButt::with_node_id_and_seeds(
-            "offline".into(),
+            outsider_addr.into(),
             HashSet::new(),
-            "offline".to_string(),
+            outsider_addr.into(),
             "another-cluster".to_string(),
             Vec::<(&str, &str)>::new(),
             FailureDetectorConfig::default(),
         );
 
+        let server_addr = "0.0.0.0:2223";
         let server = ScuttleServer::spawn(
             server_addr.into(),
             &[],
@@ -489,11 +490,14 @@ mod tests {
         syn.serialize(&mut buf);
         socket.send_to(&buf[..], server_addr).await.unwrap();
 
-        // server will drop the message, we expect the recv to timeout
         let mut buf = [0; super::UDP_MTU];
-        let resp =
-            tokio::time::timeout(Duration::from_millis(100), socket.recv_from(&mut buf)).await;
-        assert!(resp.is_err(), "unexpected response from peer");
+        let (len, _addr) = timeout(socket.recv_from(&mut buf)).await.unwrap();
+
+        let msg = ScuttleButtMessage::deserialize(&mut &buf[..len]).unwrap();
+        match msg {
+            ScuttleButtMessage::BadCluster => (),
+            message => panic!("unexpected message: {:?}", message),
+        }
 
         server.shutdown().await.unwrap();
     }