Docs: Note Mandrill's cc/bcc handling depends on preserve_recipients

Closes #391

CHANGELOG.rst

@@ -36,6 +36,16 @@ Breaking changes
 
 * Require **Django 4.0 or later** and Python 3.8 or later.
 
+Other
+~~~~~
+
+* **Mandrill (docs):** Explain how ``cc`` and ``bcc`` handling depends on
+  Mandrill's "preserve recipients" option. (Thanks to `@dgilmanAIDENTIFIED`_
+  for reporting the issue.)
+
+* **Postal (docs):** Update links to Postal's new documentation site.
+  (Thanks to `@jmduke`_.)
+
 
 v11.1
 -----
@@ -1717,6 +1727,7 @@ Features
 .. _@izimobil: https://github.com/izimobil
 .. _@janneThoft: https://github.com/janneThoft
 .. _@jc-ee: https://github.com/jc-ee
+.. _@jmduke: https://github.com/jmduke
 .. _@joshkersey: https://github.com/joshkersey
 .. _@kareemcoding: https://github.com/kareemcoding
 .. _@kika115: https://github.com/kika115

docs/esps/mandrill.rst

@@ -143,6 +143,32 @@ Limitations and quirks
   (Verified and reported to MailChimp support 4/2022;
   see `Anymail discussion #257`_ for more details.)
 
+**Cc and bcc depend on "preserve_recipients"**
+  Mandrill's handing of ``cc`` and ``bcc`` addresses depends on whether its
+  ``preserve_recipients`` option is enabled for the message.
+
+  * When preserve recipients is True, a single message is sent to all recipients.
+    The *To* and *Cc* headers list all ``to`` and ``cc`` addresses, and the message
+    is blind copied to all ``bcc`` addresses. (This is usually how people
+    expect ``cc`` and ``bcc`` to work.)
+
+  * When preserve recipients if False, Mandrill sends multiple copies of the
+    message, one per recipient. Each message has *only* that recipient's address
+    in the *To* header (even for ``cc`` and ``bcc`` addresses), so recipients
+    do not see each others' email addresses.
+
+  The default for ``preserve_recipients`` depends on Mandrill's account level
+  setting "Expose the list of recipients when sending to multiple addresses"
+  (checked sets preserve recipients to True). However, Anymail overrides this
+  setting to ``False`` for any messages that use
+  :ref:`batch sending <mandrill-templates>` features.
+
+  For individual non-batch messages, you can override your account default
+  using Anymail's :ref:`esp_extra <mandrill-esp-extra>`:
+  ``message.esp_extra = {"message": {"preserve_recipients": True}}``.
+  You can also use Anymail's :ref:`send-defaults` setting to override it for all
+  non-batch messages.
+
 **No merge headers support**
   Mandrill's API does not provide a way to support Anymail's
   :attr:`~anymail.message.AnymailMessage.merge_headers`.