Mailtrap: begin work on docs page

Author: cahna

docs/esps/mailtrap.rst

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+.. mailtrap-backend:
+
+Mailtrap
+========
+
+Anymail integrates with `Mailtrap <https://mailtrap.io/>`_'s
+transactional, bulk, or test email services, using the corresponding
+`REST API`_.
+
+.. note::
+
+    By default, Anymail connects to Mailtrap's transactional API servers.
+    If you are using Mailtrap's bulk send service, be sure to change the
+    :setting:`MAILTRAP_API_URL <ANYMAIL_MAILTRAP_API_URL>` Anymail setting
+    as shown below. Likewise, if you are using Mailtrap's test email service,
+    be sure to set :setting:`MAILTRAP_TESTING_ENABLED <ANYMAIL_MAILTRAP_TESTING_ENABLED>`
+    and :setting:`MAILTRAP_TEST_INBOX_ID <ANYMAIL_MAILTRAP_TEST_INBOX_ID>`.
+
+.. _REST API: https://api-docs.mailtrap.io/docs/mailtrap-api-docs/
+
+
+Settings
+--------
+
+.. rubric:: EMAIL_BACKEND
+
+To use Anymail's Mailtrap backend, set:
+
+  .. code-block:: python
+
+      EMAIL_BACKEND = "anymail.backends.mailtrap.EmailBackend"
+
+in your settings.py.
+
+
+.. setting:: ANYMAIL_MAILTRAP_API_TOKEN
+
+.. rubric:: MAILTRAP_API_TOKEN
+
+Required for sending:
+
+  .. code-block:: python
+
+      ANYMAIL = {
+          ...
+          "MAILTRAP_API_TOKEN": "<your API token>",
+      }
+
+Anymail will also look for ``MAILTRAP_API_TOKEN`` at the
+root of the settings file if neither ``ANYMAIL["MAILTRAP_API_TOKEN"]``
+nor ``ANYMAIL_MAILTRAP_API_TOKEN`` is set.
+
+
+.. setting:: ANYMAIL_MAILTRAP_API_URL
+
+.. rubric:: MAILTRAP_API_URL
+
+The base url for calling the Mailtrap API.
+
+The default is ``MAILTRAP_API_URL = "https://send.api.mailtrap.io/api"``, which connects
+to Mailtrap's transactional service. You must change this if you are using Mailtrap's bulk
+send service. For example, to use the bulk send service:
+
+  .. code-block:: python
+
+      ANYMAIL = {
+        "MAILTRAP_API_TOKEN": "...",
+        "MAILTRAP_API_URL": "https://bulk.api.mailtrap.io/api",
+        # ...
+      }
+
+
+.. setting:: ANYMAIL_MAILTRAP_TESTING_ENABLED
+
+.. rubric:: MAILTRAP_TESTING_ENABLED
+
+Use Mailtrap's test email service by setting this to ``True``, and providing
+:setting:`MAILTRAP_TEST_INBOX_ID <ANYMAIL_MAILTRAP_TEST_INBOX_ID>`:
+
+  .. code-block:: python
+
+      ANYMAIL = {
+        "MAILTRAP_API_TOKEN": "...",
+        "MAILTRAP_TESTING_ENABLED": True,
+        "MAILTRAP_TEST_INBOX_ID": "<your test inbox id>",
+        # ...
+      }
+
+By default, Anymail will switch to using Mailtrap's test email service API: ``https://sandbox.api.mailtrap.io/api``.
+
+.. setting:: ANYMAIL_MAILTRAP_TEST_INBOX_ID
+
+.. rubric:: MAILTRAP_TEST_INBOX_ID
+
+Required if :setting:`MAILTRAP_TESTING_ENABLED <ANYMAIL_MAILTRAP_TESTING_ENABLED>` is ``True``.
+
+
+.. _mailtrap-quirks:
+
+Limitations and quirks
+----------------------
+
+**merge_metadata unsupported**
+  Mailtrap supports :ref:`ESP stored templates <esp-stored-templates>`,
+  but does NOT support per-recipient merge data via their :ref:`batch sending <batch-send>`
+  service.
+
+
+.. _mailtrap-webhooks:
+
+Status tracking webhooks
+------------------------
+
+If you are using Anymail's normalized :ref:`status tracking <event-tracking>`, enter
+the url in the Mailtrap webhooks config for your domain. (Note that Mailtrap's sandbox domain
+does not trigger webhook events.)
+
+
+.. _About Mailtrap webhooks: https://help.mailtrap.io/article/102-webhooks
+.. _Mailtrap webhook payload: https://api-docs.mailtrap.io/docs/mailtrap-api-docs/016fe2a1efd5a-receive-events-json-format