diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 218f3502..da9a87a6 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -43,6 +43,8 @@ Fixes
 * **Mailjet:** Avoid a Mailjet API error when sending an inline image without a
   filename. (Anymail now substitutes ``"attachment"`` for the missing filename.)
   (Thanks to `@chickahoona`_ for reporting the issue.)
+* **Mailjet:** Fix a JSON parsing error on Mailjet 429 "too many requests" API
+  responses. (Thanks to `@rodrigondec`_ for reporting the issue.)
 
 
 v12.0
@@ -1779,6 +1781,7 @@ Features
 .. _@originell: https://github.com/originell
 .. _@puru02: https://github.com/puru02
 .. _@RignonNoel: https://github.com/RignonNoel
+.. _@rodrigondec: https://github.com/rodrigondec
 .. _@sblondon: https://github.com/sblondon
 .. _@scur-iolus: https://github.com/scur-iolus
 .. _@sdarwin: https://github.com/sdarwin
diff --git a/anymail/backends/mailjet.py b/anymail/backends/mailjet.py
index eb424bd4..87d0a84c 100644
--- a/anymail/backends/mailjet.py
+++ b/anymail/backends/mailjet.py
@@ -34,7 +34,10 @@ def build_message_payload(self, message, defaults):
         return MailjetPayload(message, defaults, self)
 
     def raise_for_status(self, response, payload, message):
-        if 400 <= response.status_code <= 499:
+        content_type = (
+            response.headers.get("content-type", "").split(";")[0].strip().lower()
+        )
+        if 400 <= response.status_code <= 499 and content_type == "application/json":
             # Mailjet uses 4xx status codes for partial failure in batch send;
             # we'll determine how to handle below in parse_recipient_status.
             return
diff --git a/tests/test_mailjet_backend.py b/tests/test_mailjet_backend.py
index 14c1a7e7..015a7c5c 100644
--- a/tests/test_mailjet_backend.py
+++ b/tests/test_mailjet_backend.py
@@ -10,6 +10,7 @@
 
 from anymail.exceptions import (
     AnymailAPIError,
+    AnymailRequestsAPIError,
     AnymailSerializationError,
     AnymailUnsupportedFeature,
 )
@@ -690,43 +691,41 @@ def test_mixed_status(self):
         # Mailjet's v3.1 API will partially fail a batch send, allowing valid emails
         # to go out. The API response doesn't identify the failed email addresses;
         # make sure we represent them correctly in the anymail_status.
-        response_content = json.dumps(
-            {
-                "Messages": [
-                    {
-                        "Status": "success",
-                        "CustomID": "",
-                        "To": [
-                            {
-                                "Email": "to-good@example.com",
-                                "MessageUUID": "556e896a-e041-4836-bb35-8bb75ee308c5",
-                                "MessageID": 12345678901234500,
-                                "MessageHref": "https://api.mailjet.com/v3/REST"
-                                "/message/12345678901234500",
-                            }
-                        ],
-                        "Cc": [],
-                        "Bcc": [],
-                    },
-                    {
-                        "Errors": [
-                            {
-                                "ErrorIdentifier": "f480a5a2-0334-4e08"
-                                "-b2b7-f372ce5669e0",
-                                "ErrorCode": "mj-0013",
-                                "StatusCode": 400,
-                                "ErrorMessage": '"invalid@123.4" is an invalid'
-                                " email address.",
-                                "ErrorRelatedTo": ["To[0].Email"],
-                            }
-                        ],
-                        "Status": "error",
-                    },
-                ]
-            }
-        ).encode("utf-8")
+        response_data = {
+            "Messages": [
+                {
+                    "Status": "success",
+                    "CustomID": "",
+                    "To": [
+                        {
+                            "Email": "to-good@example.com",
+                            "MessageUUID": "556e896a-e041-4836-bb35-8bb75ee308c5",
+                            "MessageID": 12345678901234500,
+                            "MessageHref": "https://api.mailjet.com/v3/REST"
+                            "/message/12345678901234500",
+                        }
+                    ],
+                    "Cc": [],
+                    "Bcc": [],
+                },
+                {
+                    "Errors": [
+                        {
+                            "ErrorIdentifier": "f480a5a2-0334-4e08"
+                            "-b2b7-f372ce5669e0",
+                            "ErrorCode": "mj-0013",
+                            "StatusCode": 400,
+                            "ErrorMessage": '"invalid@123.4" is an invalid'
+                            " email address.",
+                            "ErrorRelatedTo": ["To[0].Email"],
+                        }
+                    ],
+                    "Status": "error",
+                },
+            ]
+        }
         # Mailjet uses 400 for partial success:
-        self.set_mock_response(raw=response_content, status_code=400)
+        self.set_mock_response(json_data=response_data, status_code=400)
         msg = mail.EmailMessage(
             "Subject",
             "Message",
@@ -751,7 +750,7 @@ def test_mixed_status(self):
             msg.anymail_status.recipients["invalid@123.4"].message_id, None
         )
         self.assertEqual(msg.anymail_status.message_id, {"12345678901234500", None})
-        self.assertEqual(msg.anymail_status.esp_response.content, response_content)
+        self.assertEqual(msg.anymail_status.esp_response.json(), response_data)
 
     # noinspection PyUnresolvedReferences
     def test_send_failed_anymail_status(self):
@@ -764,6 +763,18 @@ def test_send_failed_anymail_status(self):
         self.assertEqual(self.message.anymail_status.recipients, {})
         self.assertIsNone(self.message.anymail_status.esp_response)
 
+    def test_non_json_error(self):
+        """429 (and other?) errors don't have a json payload"""
+        self.set_mock_response(
+            status_code=429, raw=b"Rate limit exceeded", content_type="text/html"
+        )
+        with self.assertRaisesRegex(
+            AnymailRequestsAPIError,
+            r"^Mailjet API response 429 .*Rate limit exceeded.*",
+        ) as cm:
+            self.message.send()
+        self.assertNotIn("Invalid JSON", str(cm.exception))
+
     # noinspection PyUnresolvedReferences
     def test_send_unparsable_response(self):
         """