Postmark: Use new RecordType field to identify event types

Simplify Postmark tracking webhook code by using new "RecordType"
field introduced with Postmark "modular webhooks". (Rather than
looking for fields that are probably only in certain events.)

Also issue configuration error on inbound url installed as tracking
webhook (and vice versa).

tests/test_postmark_webhooks.py

@@ -4,6 +4,7 @@ from datetime import datetime
 from django.utils.timezone import get_fixed_timezone, utc
 from mock import ANY
 
+from anymail.exceptions import AnymailConfigurationError
 from anymail.signals import AnymailTrackingEvent
 from anymail.webhooks.postmark import PostmarkTrackingWebhookView
 from .webhook_cases import WebhookBasicAuthTestsMixin, WebhookTestCase
@@ -202,3 +203,13 @@ class PostmarkDeliveryTestCase(WebhookTestCase):
         self.assertEqual(event.reject_reason, "spam")
         self.assertEqual(event.description, "")
         self.assertEqual(event.mta_response, "Test spam complaint details")
+
+    def test_misconfigured_inbound(self):
+        errmsg = "You seem to have set Postmark's *inbound* webhook to Anymail's Postmark *tracking* webhook URL."
+        with self.assertRaisesMessage(AnymailConfigurationError, errmsg):
+            self.client.post('/anymail/postmark/tracking/', content_type='application/json',
+                             data=json.dumps({"FromFull": {"Email": "from@example.org"}}))
+
+        with self.assertRaisesMessage(AnymailConfigurationError, errmsg):
+            self.client.post('/anymail/postmark/tracking/', content_type='application/json',
+                             data=json.dumps({"RecordType": "Inbound"}))