v0.1.0

The first release: a small daemon that lets anything on a host or in a pod
send mail through Postmark.

Sending

• Postmark-compatible HTTP API: /email, /email/batch,
  /email/withTemplate and /email/batchWithTemplates. Existing Postmark
  SDKs work by changing their base URL.
• SMTP server (POSTMARKD_SMTP_LISTEN) that converts MIME mail, including
  attachments, inline images and Postmark's X-PM-* headers.
• sendmail compatibility: invoke the binary as sendmail and cron,
  mail, PHP mail() and friends deliver through Postmark.
• postmarkd send for scripts: bodies from stdin or files, attachments,
  templates, streams, tags and metadata.
• Default From and MessageStream, plus per-request stream selection
  (?stream=, X-Postmarkd-Stream).

Reliability and scale

• Validation before Postmark sees a message; original bytes are forwarded
  untouched.
• Pooled upstream connections, a concurrency limit, and retries only when
  Postmark cannot have accepted the message (no duplicate mail).
• Optional async mode with a bounded in-memory queue, back-pressure and
  graceful drain on shutdown.

Operations

• TCP and unix socket listeners (several at once), systemd socket
  activation (including SMTP on port 25 via postmarkd-smtp.socket),
  sd_notify readiness and watchdog, and a hardened unit.
• Optional client token authentication; the Postmark token can be loaded as
  a systemd credential.
• Prometheus metrics, /healthz and /readyz; postmarkd healthcheck.
• postmarkd check-domain checks DKIM, Return-Path (SPF alignment) and
  DMARC records.
• Blackhole mode (POSTMARKD_BLACKHOLE) for tests, with captured mail at
  /blackhole/messages.

Distribution

• Linux binaries, .deb and .rpm packages for amd64 and arm64, and a
  multi-arch distroless container image at ghcr.io/rossedman/postmarkd.
• Docker Compose, Kubernetes sidecar and GitHub Actions examples.
• Documentation site at https://rossedman.io/postmarkd/.
• Benchmarks for every component, run in CI with a regression gate.