Postmark: don't error on Cc/Bcc-only send; preserve recipient caps

Postmark docs notwithstanding, Postmark allows sending mail without a
To field, as long as there is some recipient in Cc or Bcc. The API
response has a slightly different shape in this case, and Anymail now
handles that.

Also updates related recipient status parsing. Previously, Anymail's
Postmark backend converted all recipient emails to lowercase for status
reporting, and omitted Cc or Bcc recipients from
`message.anymail_status.recipients[email]`. Now, the backend preserves
the case of each recipient email as originally sent, and includes Cc
and Bcc status.

Because client code may have been relying on lowercasing recipient
emails to check status, this is a potentially breaking change.

Fixes #135

anymail/backends/postmark.py

@@ -2,7 +2,7 @@ import re
 
 from ..exceptions import AnymailRequestsAPIError
 from ..message import AnymailRecipientStatus
-from ..utils import get_anymail_setting, parse_address_list
+from ..utils import get_anymail_setting, parse_address_list, CaseInsensitiveCasePreservingDict
 
 from .base_requests import AnymailRequestsBackend, RequestsPayload
 
@@ -33,9 +33,13 @@ class EmailBackend(AnymailRequestsBackend):
             super(EmailBackend, self).raise_for_status(response, payload, message)
 
     def parse_recipient_status(self, response, payload, message):
-        # default to "unknown" status for each recipient, unless/until we find otherwise
+        # Default to "unknown" status for each recipient, unless/until we find otherwise.
+        # (This also forces recipient_status email capitalization to match that as sent,
+        # while correctly handling Postmark's lowercase-only inactive recipient reporting.)
         unknown_status = AnymailRecipientStatus(message_id=None, status='unknown')
-        recipient_status = {to.addr_spec: unknown_status for to in payload.to_emails}
+        recipient_status = CaseInsensitiveCasePreservingDict({
+            recip.addr_spec: unknown_status
+            for recip in payload.to_emails + payload.cc_and_bcc_emails})
 
         parsed_response = self.deserialize_json_response(response, payload, message)
         if not isinstance(parsed_response, list):
@@ -55,18 +59,34 @@ class EmailBackend(AnymailRequestsBackend):
             if error_code == 0:
                 # At least partial success, and (some) email was sent.
                 try:
-                    to_header = one_response["To"]
                     message_id = one_response["MessageID"]
                 except KeyError:
                     raise AnymailRequestsAPIError("Invalid Postmark API success response format",
                                                   email_message=message, payload=payload,
                                                   response=response, backend=self)
-                for to in parse_address_list(to_header):
-                    recipient_status[to.addr_spec.lower()] = AnymailRecipientStatus(
+
+                # Assume all To recipients are "sent" unless proven otherwise below.
+                # (Must use "To" from API response to get correct individual MessageIDs in batch send.)
+                try:
+                    to_header = one_response["To"]  # (missing if cc- or bcc-only send)
+                except KeyError:
+                    pass  # cc- or bcc-only send; per-recipient status not available
+                else:
+                    for to in parse_address_list(to_header):
+                        recipient_status[to.addr_spec] = AnymailRecipientStatus(
+                            message_id=message_id, status='sent')
+
+                # Assume all Cc and Bcc recipients are "sent" unless proven otherwise below.
+                # (Postmark doesn't report "Cc" or "Bcc" in API response; use original payload values.)
+                for recip in payload.cc_and_bcc_emails:
+                    recipient_status[recip.addr_spec] = AnymailRecipientStatus(
                         message_id=message_id, status='sent')
+
+                # Change "sent" to "rejected" if Postmark reported an address as "Inactive".
                 # Sadly, have to parse human-readable message to figure out if everyone got it:
                 #   "Message OK, but will not deliver to these inactive addresses: {addr_spec, ...}.
                 #    Inactive recipients are ones that have generated a hard bounce or a spam complaint."
+                # Note that error message emails are addr_spec only (no display names) and forced lowercase.
                 reject_addr_specs = self._addr_specs_from_error_msg(
                     msg, r'inactive addresses:\s*(.*)\.\s*Inactive recipients')
                 for reject_addr_spec in reject_addr_specs:
@@ -107,7 +127,7 @@ class EmailBackend(AnymailRequestsBackend):
                 raise AnymailRequestsAPIError(email_message=message, payload=payload, response=response,
                                               backend=self)
 
-        return recipient_status
+        return dict(recipient_status)
 
     @staticmethod
     def _addr_specs_from_error_msg(error_msg, pattern):
@@ -134,6 +154,7 @@ class PostmarkPayload(RequestsPayload):
         }
         self.server_token = backend.server_token  # added to headers later, so esp_extra can override
         self.to_emails = []
+        self.cc_and_bcc_emails = []  # need to track (separately) for parse_recipient_status
         self.merge_data = None
         super(PostmarkPayload, self).__init__(message, defaults, backend, headers=headers, *args, **kwargs)
 
@@ -197,6 +218,8 @@ class PostmarkPayload(RequestsPayload):
             self.data[field] = ', '.join([email.address for email in emails])
             if recipient_type == "to":
                 self.to_emails = emails
+            else:
+                self.cc_and_bcc_emails += emails
 
     def set_subject(self, subject):
         self.data["Subject"] = subject