Feature: Add envelope_sender

New EmailMessage attribute `envelope_sender` controls ESP's sender,
sending domain, or return path where supported:

* Mailgun: overrides SENDER_DOMAIN on individual message
  (domain portion only)
* Mailjet: becomes `Sender` API param
* Mandrill: becomes `return_path_domain` API param
  (domain portion only)
* SparkPost: becomes `return_path` API param
* Other ESPs: not believed to be supported

Also support undocumented Django SMTP backend behavior, where envelope
sender is given by `message.from_email` when
`message.extra_headers["From"]` is set. Fixes #91.

anymail/backends/mandrill.py

@@ -139,6 +139,10 @@ class MandrillPayload(RequestsPayload):
             "content": attachment.b64content
         })
 
+    def set_envelope_sender(self, email):
+        # Only the domain is used
+        self.data["message"]["return_path_domain"] = email.domain
+
     def set_metadata(self, metadata):
         self.data["message"]["metadata"] = metadata
 
@@ -268,6 +272,10 @@ class MandrillPayload(RequestsPayload):
         self.deprecation_warning('message.merge_vars', 'message.merge_data')
         self.set_merge_data(merge_vars)
 
+    def set_return_path_domain(self, domain):
+        self.deprecation_warning('message.return_path_domain', 'message.envelope_sender')
+        self.esp_extra.setdefault('message', {})['return_path_domain'] = domain
+
     def set_template_name(self, template_name):
         self.deprecation_warning('message.template_name', 'message.template_id')
         self.set_template_id(template_name)