diff --git a/app/mercadopago.py b/app/mercadopago.py
index 9edc10a..c0b722c 100644
--- a/app/mercadopago.py
+++ b/app/mercadopago.py
@@ -134,7 +134,15 @@ class MercadoPagoPayment:
         payment_id = str((query or {}).get('data.id') or (data.get('data') or {}).get('id') or '')
         if not payment_id.isdigit():
             return None
-        payment = self.lookup(payment_id)
+        try:
+            payment = self.lookup(payment_id)
+        except httpx.HTTPStatusError as error:
+            # Signed by Mercado Pago but about no payment of ours, such as the
+            # panel's "Simular notificação". Anything else is raised so that a
+            # real notification is retried.
+            if error.response.status_code == 404:
+                return None
+            raise
         return event_from_payment(payment, notification_id=str(data.get('id', '')))
 
 
diff --git a/deploy/portainer.env.example b/deploy/portainer.env.example
index b1d0453..3694ecb 100644
--- a/deploy/portainer.env.example
+++ b/deploy/portainer.env.example
@@ -27,6 +27,10 @@ POSTGRES_VOLUME=TBD
 OPERATOR_EMAIL=TBD
 
 PAYMENT_ADAPTER=TBD
+# With PAYMENT_ADAPTER=mercadopago. MP_ACCESS_TOKEN and MP_WEBHOOK_SECRET are
+# secrets: enter them in Portainer only, never in this file.
+MP_NOTIFICATION_URL=https://<SITE_DOMAIN>/api/payments/webhook
+MP_PUBLIC_KEY=
 FREIGHT_ADAPTER=TBD
 TINY_ADAPTER=TBD
 # Tiny API v3 application (Configurações > Geral > Aplicativos in Tiny). The
diff --git a/docker-compose.yml b/docker-compose.yml
index ce4b609..fcca18c 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -19,10 +19,20 @@ x-app-environment: &app-environment
   # this value is configured.
   OPERATOR_EMAIL: ${OPERATOR_EMAIL:-}
   OPERATOR_PASSWORD: ${OPERATOR_PASSWORD:?set OPERATOR_PASSWORD}
-  PAYMENT_ADAPTER: fake
-  # Optional: without it the webhook verifies nothing and therefore accepts
-  # nothing, which is the correct state until a provider is connected. Set it
-  # when the provider is configured, never to a value anyone could guess.
+  # fake until Mercado Pago is configured; mercadopago requires MP_ACCESS_TOKEN
+  # and MP_WEBHOOK_SECRET or the API and worker refuse to start. Test
+  # credentials (TEST-...) until the sandbox flows have passed. The card form
+  # appears only with MP_PUBLIC_KEY, and then needs PAYMENT_CSP_SOURCES too.
+  PAYMENT_ADAPTER: ${PAYMENT_ADAPTER:-fake}
+  MP_ACCESS_TOKEN: ${MP_ACCESS_TOKEN:-}
+  # The "assinatura secreta" from the webhook settings in Mercado Pago.
+  MP_WEBHOOK_SECRET: ${MP_WEBHOOK_SECRET:-}
+  # https://<SITE_DOMAIN>/api/payments/webhook, sent with every payment.
+  MP_NOTIFICATION_URL: ${MP_NOTIFICATION_URL:-}
+  MP_PUBLIC_KEY: ${MP_PUBLIC_KEY:-}
+  # The fake adapter's secret. Optional: without it the webhook verifies
+  # nothing and therefore accepts nothing, which is the correct state until a
+  # provider is connected. Never set it to a value anyone could guess.
   PAYMENT_WEBHOOK_SECRET: ${PAYMENT_WEBHOOK_SECRET:-}
   FREIGHT_ADAPTER: fake
   # Order creation in Tiny stays off until it has been tested against the
diff --git a/tests/test_mercadopago.py b/tests/test_mercadopago.py
index 277fdda..9cef52b 100644
--- a/tests/test_mercadopago.py
+++ b/tests/test_mercadopago.py
@@ -36,6 +36,10 @@ class MercadoPagoTests(unittest.TestCase):
             self.requests.append(request)
             if request.method == 'GET':
                 payment_id = request.url.path.rsplit('/', 1)[-1]
+                if payment_id == '500':
+                    return httpx.Response(500, json={'message': 'internal_error'})
+                if payment_id not in self.payments:
+                    return httpx.Response(404, json={'message': 'Payment not found'})
                 return httpx.Response(200, json=self.payments[payment_id])
             body = json.loads(request.content)
             payment = {'id': 555, 'status': 'pending', 'status_detail': 'pending_waiting_transfer',
@@ -78,6 +82,15 @@ class MercadoPagoTests(unittest.TestCase):
         self.assertEqual(self.requests[-1].headers['authorization'], 'Bearer TEST-token')
         self.assertIsNone(self.mp.parse(json.dumps({'type': 'merchant_order', 'data': {'id': '1'}}).encode()))
 
+    def test_notification_for_an_unknown_payment_is_acknowledged(self):
+        # The panel's "Simular notificação" sends a payment id that does not
+        # exist. Raising would answer 500 and Mercado Pago would retry for ever.
+        body = json.dumps({'id': 43, 'type': 'payment', 'data': {'id': '123456'}}).encode()
+        self.assertIsNone(self.mp.parse(body, {'data.id': '123456', 'type': 'payment'}))
+        # Any other failure still raises, so a real notification is retried.
+        with self.assertRaises(httpx.HTTPStatusError):
+            self.mp.parse(json.dumps({'type': 'payment', 'data': {'id': '500'}}).encode(), {'data.id': '500'})
+
     def test_amounts_outside_brl_centavos_are_not_trusted(self):
         for payment in ({'currency_id': 'USD', 'transaction_amount': 10},
                         {'currency_id': 'BRL', 'transaction_amount': 10.001},