- Support Postmark's RawEmail option;
recommend it in docs
- Handle Bcc when provided by Postmark
- Obtain `envelope_sender` from Return-Path info
Postmark now adds, rather than parsing Received-SPF
Related:
- Add `AnymailInboundMessage.bcc` convenience prop
- Test against full Postmark "check" inbound payloads
(which don't match their docs or real inbound payloads)
- Don't warn about receiving "check" payload
