diff --git a/synapse/handlers/devicemessage.py b/synapse/handlers/devicemessage.py
index 6a64c1ccf7..f6196be0a4 100644
--- a/synapse/handlers/devicemessage.py
+++ b/synapse/handlers/devicemessage.py
@@ -42,7 +42,14 @@ from synapse.logging.opentracing import (
     log_kv,
     set_tag,
 )
-from synapse.types import JsonDict, Requester, StreamKeyType, UserID, get_domain_from_id
+from synapse.types import (
+    JsonDict,
+    Requester,
+    StreamKeyType,
+    UserID,
+    get_domain_from_id,
+    is_compliant_user_id,
+)
 from synapse.util import split_dict_to_fit_to_size
 from synapse.util.json import json_encoder
 from synapse.util.stringutils import random_string_insecure_fast
@@ -122,6 +129,14 @@ class DeviceMessageHandler:
                 sender_user_id,
             )
             return
+
+        if not is_compliant_user_id(sender_user_id):
+            logger.warning(
+                "Dropping device message from %r with non-compliant sender %r",
+                origin,
+                sender_user_id,
+            )
+            return
         message_type = content["type"]
         message_id = content["message_id"]
         for user_id, by_device in content["messages"].items():
diff --git a/tests/rest/client/test_sendtodevice.py b/tests/rest/client/test_sendtodevice.py
index e9b607cf23..21aa754cea 100644
--- a/tests/rest/client/test_sendtodevice.py
+++ b/tests/rest/client/test_sendtodevice.py
@@ -543,6 +543,48 @@ class SendToDeviceTestCase(HomeserverTestCase):
             ],
         )
 
+    def test_remote_non_compliant_sender(self) -> None:
+        """
+        Tests that a to-device message from a non-compliant (grandfathered
+        historical) sender is dropped, while one from a compliant sender on the
+        same server is delivered.
+        """
+        user2 = self.register_user("u2", "pass")
+        user2_tok = self.login("u2", "pass", "d2")
+
+        federation_registry = self.hs.get_federation_registry()
+
+        for sender, content in (
+            ("@héllo:example.org", {"foo": "bar"}),
+            ("@user:example.org", {"hiss": "meow"}),
+        ):
+            self.get_success(
+                federation_registry.on_edu(
+                    EduTypes.DIRECT_TO_DEVICE,
+                    "example.org",
+                    {
+                        "sender": sender,
+                        "type": "org.example.test",
+                        "messages": {user2: {"d2": content}},
+                        "message_id": "1",
+                    },
+                )
+            )
+
+        channel = self.make_request("GET", "/sync", access_token=user2_tok)
+        self.assertEqual(channel.code, 200, channel.result)
+        messages = channel.json_body.get("to_device", {}).get("events", [])
+        self.assertEqual(
+            messages,
+            [
+                {
+                    "content": {"hiss": "meow"},
+                    "sender": "@user:example.org",
+                    "type": "org.example.test",
+                }
+            ],
+        )
+
     def test_limited_sync(self) -> None:
         """If a limited sync for to-devices happens the next /sync should respond immediately."""