diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index e679b1a..34ec0fe 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -41,6 +41,11 @@ Fixes
 * **Postmark:** Fix two different errors when sending with a template but no merge
   data. (Thanks to `@kareemcoding`_ and `@Tobeyforce`_ for reporting them.)
 
+* **Postmark:** Fix silent failure when sending with long metadata keys and some
+  other errors Postmark detects at send time. Report invalid 'cc' and 'bcc' addresses
+  detected at send time the same as 'to' recipients. (Thanks to `@chrisgrande`_ for
+  reporting the problem.)
+
 
 v8.2
 -----
@@ -1222,6 +1227,7 @@ Features
 .. _@alee: https://github.com/alee
 .. _@anstosa: https://github.com/anstosa
 .. _@calvin: https://github.com/calvin
+.. _@chrisgrande: https://github.com/chrisgrande
 .. _@costela: https://github.com/costela
 .. _@decibyte: https://github.com/decibyte
 .. _@dominik-lekse: https://github.com/dominik-lekse
diff --git a/anymail/backends/postmark.py b/anymail/backends/postmark.py
index 05535e2..277041f 100644
--- a/anymail/backends/postmark.py
+++ b/anymail/backends/postmark.py
@@ -94,22 +94,24 @@ class EmailBackend(AnymailRequestsBackend):
                         message_id=None, status='rejected')
 
             elif error_code == 300:  # Invalid email request
-                # Either the From address or at least one recipient was invalid. Email not sent.
+                # Various parse-time validation errors, which may include invalid recipients. Email not sent.
                 # response["To"] is not populated for this error; must examine response["Message"]:
-                #   "Invalid 'To' address: '{addr_spec}'."
-                #   "Error parsing 'To': Illegal email domain '{domain}' in address '{addr_spec}'."
-                #   "Error parsing 'To': Illegal email address '{addr_spec}'. It must contain the '@' symbol."
-                #   "Invalid 'From' address: '{email_address}'."
-                if "'From' address" in msg:
-                    # Normal error
-                    raise AnymailRequestsAPIError(email_message=message, payload=payload, response=response,
-                                                  backend=self)
-                else:
-                    # Use AnymailRecipientsRefused logic
+                if re.match(r"^(Invalid|Error\s+parsing)\s+'(To|Cc|Bcc)'", msg, re.IGNORECASE):
+                    # Recipient-related errors: use AnymailRecipientsRefused logic
+                    #   "Invalid 'To' address: '{addr_spec}'."
+                    #   "Error parsing 'Cc': Illegal email domain '{domain}' in address '{addr_spec}'."
+                    #   "Error parsing 'Bcc': Illegal email address '{addr_spec}'. It must contain the '@' symbol."
                     invalid_addr_specs = self._addr_specs_from_error_msg(msg, r"address:?\s*'(.*)'")
                     for invalid_addr_spec in invalid_addr_specs:
                         recipient_status[invalid_addr_spec] = AnymailRecipientStatus(
                             message_id=None, status='invalid')
+                else:
+                    # Non-recipient errors; handle as normal API error response
+                    #   "Invalid 'From' address: '{email_address}'."
+                    #   "Error parsing 'Reply-To': Illegal email domain '{domain}' in address '{addr_spec}'."
+                    #   "Invalid metadata content. ..."
+                    raise AnymailRequestsAPIError(email_message=message, payload=payload,
+                                                  response=response, backend=self)
 
             elif error_code == 406:  # Inactive recipient
                 # All recipients were rejected as hard-bounce or spam-complaint. Email not sent.
diff --git a/tests/test_postmark_backend.py b/tests/test_postmark_backend.py
index 9c0fc42..449685d 100644
--- a/tests/test_postmark_backend.py
+++ b/tests/test_postmark_backend.py
@@ -709,6 +709,19 @@ class PostmarkBackendRecipientsRefusedTests(PostmarkBackendMockAPITestCase):
         status = msg.anymail_status
         self.assertEqual(status.recipients['Invalid@LocalHost'].status, 'invalid')
 
+    def test_recipients_parse_error(self):
+        self.set_mock_response(
+            status_code=422,
+            raw=b"""{
+              "ErrorCode": 300,
+              "Message": "Error parsing 'Cc': Illegal email domain '+' in address 'user@+'."
+            }""")
+        msg = mail.EmailMessage('Subject', 'Body', 'from@example.com', cc=["user@+"])
+        with self.assertRaises(AnymailRecipientsRefused):
+            msg.send()
+        status = msg.anymail_status
+        self.assertEqual(status.recipients['user@+'].status, 'invalid')
+
     def test_from_email_invalid(self):
         # Invalid 'From' address generates same Postmark ErrorCode 300 as invalid 'To',
         # but should raise a different Anymail error
@@ -720,6 +733,33 @@ class PostmarkBackendRecipientsRefusedTests(PostmarkBackendMockAPITestCase):
         with self.assertRaises(AnymailAPIError):
             msg.send()
 
+    def test_reply_to_invalid(self):
+        # Make sure 'Reply-To' error doesn't get caught in invalid 'To' logic:
+        self.set_mock_response(
+            status_code=422,
+            raw=b"""{
+                "ErrorCode": 300,
+                "Message": "Error parsing 'Reply-To': Illegal email domain '+' in address 'invalid@+'."}
+            """)
+        msg = mail.EmailMessage('Subject', 'Body', 'from@example.com', ['to@example.com'],
+                                reply_to=["invalid@+"])
+        with self.assertRaisesMessage(AnymailAPIError, "Error parsing 'Reply-To'"):
+            msg.send()
+
+    def test_errorcode_300(self):
+        # Various other problems generate same Postmark ErrorCode 300 as invalid 'To',
+        # but should raise ordinary API errors
+        self.set_mock_response(
+            status_code=422,
+            raw=b"""{
+                "ErrorCode": 300,
+                "Message": "Invalid metadata content. Field names are limited to 20 characters..."
+            }""")
+        msg = mail.EmailMessage('Subject', 'Body', 'from@example.com', ['to@example.com'])
+        msg.metadata = {"this-key-name-is-too-long": "data"}
+        with self.assertRaisesMessage(AnymailAPIError, "Invalid metadata content"):
+            msg.send()
+
     def test_fail_silently(self):
         self.set_mock_response(
             status_code=422,