Using undeliverable @example.com recipient addresses leads some ESPs
to flag the Anymail test accounts. Switch all live integration tests
to mailinator.com recipients (unless they were already using the ESP's
own "test sink" addresses).
* Don't send *quite* so many emails during live integration tests.
(Our test account is throttled to 40/hour.)
* Relax message_id check in integration tests. SendinBlue appears
to use both @smtp-relay.mail.fr and @smtp-relay.sendinblue.com
Message-IDs.
* Note requirement for HTML message body in docs.
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.)
