diff --git a/gateway/platforms/msgraph_webhook.py b/gateway/platforms/msgraph_webhook.py
index 9c6feabad8..9b8e01f6a1 100644
--- a/gateway/platforms/msgraph_webhook.py
+++ b/gateway/platforms/msgraph_webhook.py
@@ -5,6 +5,7 @@ from __future__ import annotations
 import asyncio
 import json
 import logging
+from collections import deque
 from hashlib import sha1
 from typing import Any, Awaitable, Callable, Dict, Optional
 
@@ -29,6 +30,7 @@ logger = logging.getLogger(__name__)
 DEFAULT_HOST = "0.0.0.0"
 DEFAULT_PORT = 8646
 DEFAULT_WEBHOOK_PATH = "/msgraph/webhook"
+DEFAULT_MAX_SEEN_RECEIPTS = 5000
 NotificationScheduler = Callable[[Dict[str, Any], MessageEvent], Awaitable[None] | None]
 
 
@@ -55,9 +57,13 @@ class MSGraphWebhookAdapter(BasePlatformAdapter):
             if str(value).strip()
         ]
         self._client_state: Optional[str] = self._string_or_none(extra.get("client_state"))
+        self._max_seen_receipts = max(
+            1, int(extra.get("max_seen_receipts", DEFAULT_MAX_SEEN_RECEIPTS))
+        )
         self._runner = None
         self._notification_scheduler: Optional[NotificationScheduler] = None
         self._seen_receipts: set[str] = set()
+        self._seen_receipt_order: deque[str] = deque()
         self._accepted_count = 0
         self._duplicate_count = 0
 
@@ -172,10 +178,10 @@ class MSGraphWebhookAdapter(BasePlatformAdapter):
                 continue
 
             receipt_key = self._build_receipt_key(notification)
-            if receipt_key in self._seen_receipts:
+            if self._has_seen_receipt(receipt_key):
                 duplicates += 1
                 continue
-            self._seen_receipts.add(receipt_key)
+            self._remember_receipt(receipt_key)
 
             accepted += 1
             scheduled += 1
@@ -213,6 +219,16 @@ class MSGraphWebhookAdapter(BasePlatformAdapter):
         provided = self._string_or_none(notification.get("clientState"))
         return provided == expected
 
+    def _has_seen_receipt(self, receipt_key: str) -> bool:
+        return receipt_key in self._seen_receipts
+
+    def _remember_receipt(self, receipt_key: str) -> None:
+        self._seen_receipts.add(receipt_key)
+        self._seen_receipt_order.append(receipt_key)
+        while len(self._seen_receipt_order) > self._max_seen_receipts:
+            oldest = self._seen_receipt_order.popleft()
+            self._seen_receipts.discard(oldest)
+
     def _build_message_event(
         self,
         notification: Dict[str, Any],
diff --git a/tests/gateway/test_msgraph_webhook.py b/tests/gateway/test_msgraph_webhook.py
index a281603a3c..404156c45f 100644
--- a/tests/gateway/test_msgraph_webhook.py
+++ b/tests/gateway/test_msgraph_webhook.py
@@ -185,3 +185,41 @@ class TestMSGraphNotifications:
         await asyncio.sleep(0.05)
 
         assert len(scheduled) == 1
+
+    @pytest.mark.anyio
+    async def test_seen_receipts_are_bounded(self):
+        adapter = _make_adapter(max_seen_receipts=2)
+
+        async def _capture(notification, event):
+            return None
+
+        adapter.set_notification_scheduler(_capture)
+
+        async def _post(notification_id: str):
+            payload = {
+                "value": [
+                    {
+                        "id": notification_id,
+                        "subscriptionId": "sub-1",
+                        "changeType": "updated",
+                        "resource": "communications/onlineMeetings/meeting-3",
+                        "clientState": "expected-client-state",
+                    }
+                ]
+            }
+            return await adapter._handle_notification(_FakeRequest(json_payload=payload))
+
+        first = await _post("notif-a")
+        second = await _post("notif-b")
+        third = await _post("notif-c")
+
+        assert first.status == 202
+        assert second.status == 202
+        assert third.status == 202
+        assert len(adapter._seen_receipts) == 2
+        assert list(adapter._seen_receipt_order) == ["id:notif-b", "id:notif-c"]
+
+        replay = await _post("notif-a")
+        replay_data = json.loads(replay.text)
+        assert replay_data["accepted"] == 1
+        assert replay_data["duplicates"] == 0