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.
Add support for sending transactional email through SendinBlue. (Thanks to @RignonNoel.)
Partially implements #84. (Tracking webhooks will be a separate PR. SendinBlue doesn't support inbound handling.)
* message.template_id to use ESP stored templates
* message.merge_data and merge_global_data
to supply per-recipient/global merge variables
(with or without an ESP stored template)
* When using per-recipient merge_data, tell ESP to use
batch send: individual message per "to" address.
(Mailgun does this automatically; SendGrid requires
using a different "to" field; Mandrill requires
`preserve_recipients=False`; Postmark doesn't
support *this type* of batch sending with merge data.)
* Allow message.from_email=None (must be set after
init) and message.subject=None to suppress those
fields in API calls (for ESPs that allow "From" and
"Subject" in their template definitions).
Mailgun:
* Emulate merge_global_data by copying to
recipient-variables for each recipient.
SendGrid:
* Add delimiters to merge field names via
esp_extra['merge_field_format'] or
ANYMAIL_SENDGRID_MERGE_FIELD_FORMAT setting.
Mandrill:
* Remove Djrill versions of these features;
update migration notes.
Closes#5.
