diff --git a/anymail/webhooks/postmark.py b/anymail/webhooks/postmark.py
index e6824b92..652947d7 100644
--- a/anymail/webhooks/postmark.py
+++ b/anymail/webhooks/postmark.py
@@ -1,8 +1,9 @@
 import json
+import warnings
 
 from django.utils.dateparse import parse_datetime
 
-from ..exceptions import AnymailConfigurationError
+from ..exceptions import AnymailConfigurationError, AnymailWarning
 from ..inbound import AnymailInboundMessage
 from ..signals import (
     AnymailInboundEvent,
@@ -183,6 +184,18 @@ def esp_to_anymail_event(self, esp_event):
             for attachment in esp_event.get("Attachments", [])
         ]
 
+        # Warning to the user regarding the workaround of above.
+        for attachment in esp_event.get("Attachments", []):
+            if "Data" in attachment:
+                warnings.warn(
+                    "Received a test webhook attachment. "
+                    "It is recommended to test with real inbound events. "
+                    "See https://github.com/anymail/django-anymail/issues/304 "
+                    "for more information.",
+                    AnymailWarning,
+                )
+                break
+
         message = AnymailInboundMessage.construct(
             from_email=self._address(esp_event.get("FromFull")),
             to=", ".join([self._address(to) for to in esp_event.get("ToFull", [])]),
diff --git a/tests/test_postmark_inbound.py b/tests/test_postmark_inbound.py
index 47a92b22..7dd8d4b5 100644
--- a/tests/test_postmark_inbound.py
+++ b/tests/test_postmark_inbound.py
@@ -4,7 +4,7 @@
 
 from django.test import tag
 
-from anymail.exceptions import AnymailConfigurationError
+from anymail.exceptions import AnymailConfigurationError, AnymailWarning
 from anymail.inbound import AnymailInboundMessage
 from anymail.signals import AnymailInboundEvent
 from anymail.webhooks.postmark import PostmarkInboundWebhookView
@@ -178,11 +178,14 @@ def test_attachments(self):
             ]
         }
 
-        response = self.client.post(
-            "/anymail/postmark/inbound/",
-            content_type="application/json",
-            data=json.dumps(raw_event),
-        )
+        with self.assertWarnsRegex(
+            AnymailWarning, r"Received a test webhook attachment. "
+        ):
+            response = self.client.post(
+                "/anymail/postmark/inbound/",
+                content_type="application/json",
+                data=json.dumps(raw_event),
+            )
         self.assertEqual(response.status_code, 200)
         kwargs = self.assert_handler_called_once_with(
             self.inbound_handler,