MailerSend: support extra headers

MailerSend added a `"headers"` API field
(which is available to "Enterprise
accounts only").

CHANGELOG.rst

@@ -55,6 +55,9 @@ Features
   and ``tags`` when sending with a ``template_id``.
   (Requires boto3 v1.34.98 or later.)
 
+* **MailerSend:** Allow all extra headers. (Note that MailerSend limits use
+  of this feature to "Enterprise accounts only.")
+
 Fixes
 ~~~~~
 

anymail/backends/mailersend.py

@@ -242,8 +242,8 @@ class MailerSendPayload(RequestsPayload):
             self.data["reply_to"] = self.make_mailersend_email(emails[0])
 
     def set_extra_headers(self, headers):
-        # MailerSend doesn't support arbitrary email headers, but has
+        # MailerSend has individual API params for In-Reply-To and Precedence: bulk.
-        # individual API params for In-Reply-To and Precedence: bulk.
+        # The general "headers" option "is available to Enterprise accounts only".
         # (headers is a CaseInsensitiveDict, and is a copy so safe to modify.)
         in_reply_to = headers.pop("In-Reply-To", None)
         if in_reply_to is not None:
@@ -256,7 +256,9 @@ class MailerSendPayload(RequestsPayload):
             self.data["precedence_bulk"] = is_bulk
 
         if headers:
-            self.unsupported_feature("most extra_headers (see docs)")
+            self.data["headers"] = [
+                {"name": field, "value": value} for field, value in headers.items()
+            ]
 
     def set_text_body(self, body):
         self.data["text"] = body

docs/esps/mailersend.rst

@@ -204,7 +204,12 @@ see :ref:`unsupported-features`.
   Anymail will use only the first one.
 
 **Limited extra headers**
-  MailerSend does not allow most extra headers. There are two exceptions:
+  MailerSend allows extra email headers for "Enterprise accounts only."
+  If you try to send :ref:`extra headers <message-headers>` with a non-enterprise
+  account, you may receive an API error.
+
+  However, MailerSend has special handling for two headers,
+  and *any* MailerSend account can send messages with them:
 
   * You can include :mailheader:`In-Reply-To` in extra headers, set to
     a message-id (without the angle brackets).
@@ -216,7 +221,10 @@ see :ref:`unsupported-features`.
     if your extra headers have :mailheader:`Precedence` set to ``"bulk"`` or
     ``"list"`` or ``"junk"``, or ``false`` for any other value.
 
-  Any other extra headers will raise an
+  .. versionchanged:: 11.0
+
+      In earlier releases, attempting to send other headers
+      (even with an enterprise account) would raise an
       :exc:`~anymail.exceptions.AnymailUnsupportedFeature` error.
 
 **No merge headers support**

tests/test_mailersend_backend.py

@@ -145,23 +145,6 @@ class MailerSendBackendStandardEmailTests(MailerSendBackendMockAPITestCase):
         )
 
     def test_custom_headers(self):
-        email = mail.EmailMessage(
-            "Subject",
-            "Body goes here",
-            "from@example.com",
-            ["to1@example.com"],
-            headers={
-                "Reply-To": "another@example.com",
-                "In-Reply-To": "12345@example.com",
-                "X-MyHeader": "my value",
-                "Message-ID": "mycustommsgid@example.com",
-                "Precedence": "Bulk",
-            },
-        )
-        with self.assertRaisesMessage(AnymailUnsupportedFeature, "extra_headers"):
-            email.send()
-
-    def test_supported_custom_headers(self):
         email = mail.EmailMessage(
             "Subject",
             "Body goes here",
@@ -171,13 +154,20 @@ class MailerSendBackendStandardEmailTests(MailerSendBackendMockAPITestCase):
                 "Reply-To": "another@example.com",
                 "In-Reply-To": "12345@example.com",
                 "Precedence": "Bulk",
+                # Other custom headers only available to enterprise accounts:
+                "X-Custom": "custom header",
             },
         )
         email.send()
         data = self.get_api_call_json()
+        # Special handling headers:
         self.assertEqual(data["reply_to"], {"email": "another@example.com"})
         self.assertEqual(data["in_reply_to"], "12345@example.com")
         self.assertIs(data["precedence_bulk"], True)
+        # Other headers:
+        self.assertEqual(
+            data["headers"], [{"name": "X-Custom", "value": "custom header"}]
+        )
 
     def test_html_message(self):
         text_content = "This is an important message."
@@ -607,6 +597,7 @@ class MailerSendBackendAnymailFeatureTests(MailerSendBackendMockAPITestCase):
         self.assertNotIn("reply_to", data)
         self.assertNotIn("html", data)
         self.assertNotIn("attachments", data)
+        self.assertNotIn("headers", data)
         self.assertNotIn("template_id", data)
         self.assertNotIn("tags", data)
         self.assertNotIn("variables", data)

tests/test_mailersend_integration.py

@@ -81,8 +81,13 @@ class MailerSendBackendIntegrationTests(AnymailTestMixin, SimpleTestCase):
             bcc=["test+bcc1@anymail.dev", "Blind Copy 2 <test+bcc2@anymail.dev>"],
             # MailerSend only supports single reply_to:
             reply_to=["Reply <reply@example.com>"],
-            # MailerSend supports very limited extra headers:
+            headers={
-            headers={"Precedence": "bulk", "In-Reply-To": "earlier-id@anymail.dev"},
+                # Special handling for these headers (available to all accounts):
+                "Precedence": "bulk",
+                "In-Reply-To": "earlier-id@anymail.dev",
+                # Only "enterprise accounts" can send other custom headers:
+                "X-Custom": "anymail test",
+            },
             send_at=send_at,
             tags=["tag 1", "tag 2"],
             track_clicks=False,