Mailgun, SparkPost: support multiple from_email addresses

[RFC-5322 allows](https://tools.ietf.org/html/rfc5322#section-3.6.2)
multiple addresses in the From header.

Django's SMTP backend supports this, as a single comma-separated
string (*not* a list of strings like the recipient params):

    from_email='one@example.com, two@example.com'
    to=['one@example.com', 'two@example.com']

Both Mailgun and SparkPost support multiple From addresses
(and Postmark accepts them, though truncates to the first one
on their end). For compatibility with Django -- and because
Anymail attempts to support all ESP features -- Anymail now
allows multiple From addresses, too, for ESPs that support it.

Note: as a practical matter, deliverability with multiple
From addresses is pretty bad. (Google outright rejects them.)

This change also reworks Anymail's internal ParsedEmail object,
and approach to parsing addresses, for better consistency with
Django's SMTP backend and improved error messaging.

In particular, Django (and now Anymail) allows multiple email
addresses in a single recipient string:

    to=['one@example.com', 'two@example.com, three@example.com']
    len(to) == 2  # but there will be three recipients

Fixes #60

anymail/backends/base.py

@@ -8,7 +8,7 @@ from django.utils.timezone import is_naive, get_current_timezone, make_aware, ut
 from ..exceptions import AnymailCancelSend, AnymailError, AnymailUnsupportedFeature, AnymailRecipientsRefused
 from ..message import AnymailStatus
 from ..signals import pre_send, post_send
-from ..utils import (Attachment, ParsedEmail, UNSET, combine, last, get_anymail_setting,
+from ..utils import (Attachment, UNSET, combine, last, get_anymail_setting, parse_address_list,
                      force_non_lazy, force_non_lazy_list, force_non_lazy_dict, is_lazy)
 
 
@@ -216,12 +216,12 @@ class BasePayload(object):
     # the combined/converted results for each attr.
     base_message_attrs = (
         # Standard EmailMessage/EmailMultiAlternatives props
-        ('from_email', last, 'parsed_email'),
-        ('to', combine, 'parsed_emails'),
-        ('cc', combine, 'parsed_emails'),
-        ('bcc', combine, 'parsed_emails'),
+        ('from_email', last, parse_address_list),  # multiple from_emails are allowed
+        ('to', combine, parse_address_list),
+        ('cc', combine, parse_address_list),
+        ('bcc', combine, parse_address_list),
         ('subject', last, force_non_lazy),
-        ('reply_to', combine, 'parsed_emails'),
+        ('reply_to', combine, parse_address_list),
         ('extra_headers', combine, force_non_lazy_dict),
         ('body', last, force_non_lazy),  # special handling below checks message.content_subtype
         ('alternatives', combine, 'prepped_alternatives'),
@@ -266,6 +266,8 @@ class BasePayload(object):
             if value is not UNSET:
                 if attr == 'body':
                     setter = self.set_html_body if message.content_subtype == 'html' else self.set_text_body
+                elif attr == 'from_email':
+                    setter = self.set_from_email_list
                 else:
                     # AttributeError here? Your Payload subclass is missing a set_<attr> implementation
                     setter = getattr(self, 'set_%s' % attr)
@@ -300,14 +302,6 @@ class BasePayload(object):
     # Attribute converters
     #
 
-    def parsed_email(self, address):
-        return ParsedEmail(address, self.message.encoding)  # (handles lazy address)
-
-    def parsed_emails(self, addresses):
-        encoding = self.message.encoding
-        return [ParsedEmail(address, encoding)  # (handles lazy address)
-                for address in addresses]
-
     def prepped_alternatives(self, alternatives):
         return [(force_non_lazy(content), mimetype)
                 for (content, mimetype) in alternatives]
@@ -348,8 +342,17 @@ class BasePayload(object):
         raise NotImplementedError("%s.%s must implement init_payload" %
                                   (self.__class__.__module__, self.__class__.__name__))
 
+    def set_from_email_list(self, emails):
+        # If your backend supports multiple from emails, override this to handle the whole list;
+        # otherwise just implement set_from_email
+        if len(emails) > 1:
+            self.unsupported_feature("multiple from emails")
+            # fall through if ignoring unsupported features
+        if len(emails) > 0:
+            self.set_from_email(emails[0])
+
     def set_from_email(self, email):
-        raise NotImplementedError("%s.%s must implement set_from_email" %
+        raise NotImplementedError("%s.%s must implement set_from_email or set_from_email_list" %
                                   (self.__class__.__module__, self.__class__.__name__))
 
     def set_to(self, emails):

anymail/backends/mailgun.py

@@ -124,15 +124,12 @@ class MailgunPayload(RequestsPayload):
         self.data = {}   # {field: [multiple, values]}
         self.files = []  # [(field, multiple), (field, values)]
 
-    def set_from_email(self, email):
-        self.data["from"] = str(email)
-        if self.sender_domain is None:
-            # try to intuit sender_domain from from_email
-            try:
-                _, domain = email.email.split('@')
-                self.sender_domain = domain
-            except ValueError:
-                pass
+    def set_from_email_list(self, emails):
+        # Mailgun supports multiple From email addresses
+        self.data["from"] = [email.address for email in emails]
+        if self.sender_domain is None and len(emails) > 0:
+            # try to intuit sender_domain from first from_email
+            self.sender_domain = emails[0].domain or None
 
     def set_recipients(self, recipient_type, emails):
         assert recipient_type in ["to", "cc", "bcc"]

anymail/backends/postmark.py

@@ -138,8 +138,10 @@ class PostmarkPayload(RequestsPayload):
     def init_payload(self):
         self.data = {}   # becomes json
 
-    def set_from_email(self, email):
-        self.data["From"] = email.address
+    def set_from_email_list(self, emails):
+        # Postmark accepts multiple From email addresses
+        # (though truncates to just the first, on their end, as of 4/2017)
+        self.data["From"] = ", ".join([email.address for email in emails])
 
     def set_recipients(self, recipient_type, emails):
         assert recipient_type in ["to", "cc", "bcc"]

anymail/backends/sendgrid.py

@@ -7,7 +7,7 @@ from requests.structures import CaseInsensitiveDict
 from .base_requests import AnymailRequestsBackend, RequestsPayload
 from ..exceptions import AnymailConfigurationError, AnymailRequestsAPIError, AnymailWarning, AnymailDeprecationWarning
 from ..message import AnymailRecipientStatus
-from ..utils import get_anymail_setting, timestamp, update_deep
+from ..utils import get_anymail_setting, timestamp, update_deep, parse_address_list
 
 
 class EmailBackend(AnymailRequestsBackend):
@@ -115,7 +115,7 @@ class SendGridPayload(RequestsPayload):
         if "Reply-To" in headers:
             # Reply-To must be in its own param
             reply_to = headers.pop('Reply-To')
-            self.set_reply_to([self.parsed_email(reply_to)])
+            self.set_reply_to(parse_address_list([reply_to]))
         if len(headers) > 0:
             self.data["headers"] = dict(headers)  # flatten to normal dict for json serialization
         else:

anymail/backends/sparkpost.py

@@ -129,8 +129,10 @@ class SparkPostPayload(BasePayload):
 
         return self.params
 
-    def set_from_email(self, email):
-        self.params['from_email'] = email.address
+    def set_from_email_list(self, emails):
+        # SparkPost supports multiple From email addresses,
+        # as a single comma-separated string
+        self.params['from_email'] = ", ".join([email.address for email in emails])
 
     def set_to(self, emails):
         if emails: