diff --git a/hyperactor/src/mailbox.rs b/hyperactor/src/mailbox.rs
index 4a622add8..2b31b8124 100644
--- a/hyperactor/src/mailbox.rs
+++ b/hyperactor/src/mailbox.rs
@@ -324,6 +324,13 @@ impl MessageEnvelope {
         self.errors.push(error)
     }
 
+    /// Change the sender on the envelope in case it was set incorrectly. This
+    /// should only be used by CommActor since it is forwarding from another
+    /// sender.
+    pub fn update_sender(&mut self, sender: ActorId) {
+        self.sender = sender;
+    }
+
     /// The message has been determined to be undeliverable with the
     /// provided error. Mark the envelope with the error and return to
     /// sender.
diff --git a/monarch_hyperactor/src/actor.rs b/monarch_hyperactor/src/actor.rs
index 5256138d7..ab07af6a7 100644
--- a/monarch_hyperactor/src/actor.rs
+++ b/monarch_hyperactor/src/actor.rs
@@ -20,12 +20,17 @@ use hyperactor::Handler;
 use hyperactor::Instance;
 use hyperactor::Named;
 use hyperactor::OncePortHandle;
+use hyperactor::ProcId;
 use hyperactor::mailbox::MessageEnvelope;
 use hyperactor::mailbox::Undeliverable;
 use hyperactor::message::Bind;
 use hyperactor::message::Bindings;
 use hyperactor::message::Unbind;
+use hyperactor::reference::WorldId;
+use hyperactor_mesh::actor_mesh::CAST_ACTOR_MESH_ID;
+use hyperactor_mesh::comm::multicast::CAST_ORIGINATING_SENDER;
 use hyperactor_mesh::comm::multicast::CastInfo;
+use hyperactor_mesh::reference::ActorMeshId;
 use monarch_types::PickledPyObject;
 use monarch_types::SerializablePyErr;
 use pyo3::IntoPyObjectExt;
@@ -493,6 +498,47 @@ impl PythonActor {
     }
 }
 
+/// An undeliverable might have its sender address set as the comm actor instead
+/// of the original sender. Update it based on the headers present in the message
+/// so it matches the sender.
+fn update_undeliverable_envelope_for_casting(
+    mut envelope: Undeliverable<MessageEnvelope>,
+) -> Undeliverable<MessageEnvelope> {
+    let old_actor = envelope.0.sender().clone();
+    // v1 casting
+    if let Some(actor_id) = envelope.0.headers().get(CAST_ORIGINATING_SENDER).cloned() {
+        tracing::debug!(
+            actor_id = %old_actor,
+            "PythonActor::handle_undeliverable_message: remapped comm-actor id to id from CAST_ORIGINATING_SENDER {}", actor_id
+        );
+        envelope.0.update_sender(actor_id);
+    // v0 casting
+    } else if let Some(actor_mesh_id) = envelope.0.headers().get(CAST_ACTOR_MESH_ID) {
+        match actor_mesh_id {
+            ActorMeshId::V0(proc_mesh_id, actor_name) => {
+                let actor_id = ActorId(
+                    ProcId::Ranked(WorldId(proc_mesh_id.0.clone()), 0),
+                    actor_name.clone(),
+                    0,
+                );
+                tracing::debug!(
+                    actor_id = %old_actor,
+                    "PythonActor::handle_undeliverable_message: remapped comm-actor id to mesh id from CAST_ACTOR_MESH_ID {}", actor_id
+                );
+                envelope.0.update_sender(actor_id);
+            }
+            ActorMeshId::V1(_) => {
+                tracing::debug!(
+                    "PythonActor::handle_undeliverable_message: headers present but V1 ActorMeshId; leaving actor_id unchanged"
+                );
+            }
+        }
+    } else {
+        // Do nothing, it wasn't from a comm actor.
+    }
+    envelope
+}
+
 #[async_trait]
 impl Actor for PythonActor {
     type Params = PickledPyObject;
@@ -537,9 +583,21 @@ impl Actor for PythonActor {
     async fn handle_undeliverable_message(
         &mut self,
         ins: &Instance<Self>,
-        envelope: Undeliverable<MessageEnvelope>,
+        mut envelope: Undeliverable<MessageEnvelope>,
     ) -> Result<(), anyhow::Error> {
-        assert_eq!(envelope.0.sender(), ins.self_id());
+        if envelope.0.sender() != ins.self_id() {
+            // This can happen if the sender is comm. Update the envelope.
+            envelope = update_undeliverable_envelope_for_casting(envelope);
+        }
+        assert_eq!(
+            envelope.0.sender(),
+            ins.self_id(),
+            "undeliverable message was returned to the wrong actor. \
+            Return address = {}, src actor = {}, dest actor port = {}",
+            envelope.0.sender(),
+            ins.self_id(),
+            envelope.0.dest()
+        );
 
         let cx = Context::new(ins, envelope.0.headers().clone());