[Kayvan-Zahiri]

Problem and change

Django's Postmark backend leaves an automatically opened client attached after send_batch raises or returns a failed message. The close call is only reached after every batch succeeds.

Put the batch loop inside try/finally so clients opened by send_messages are closed on failure too. Keep this cleanup inside the existing exception-handling boundary so fail_silently, error signals, partial success counts, and caller-owned connection reuse remain intact. Add an Unreleased changelog entry.

Fixes #38.

Validation

• Before: 4 failed, 4 passed new lifecycle cases. Cover transport exceptions and per-message failures, both fail_silently values, and automatically opened versus caller-owned clients.
• After: full pytest --cov=postmark --cov-report=term --cov-fail-under=85 — 462 passed, 96.32% coverage.
• Full Ruff lint and format checks pass; MyPy passes across 64 source files.
• Required pre-commit run --all-files — all applicable hooks pass, including Poetry metadata/lock consistency, pinned Ruff/MyPy, codespell, Bandit, and secret scanning.
• git diff --check passes. No unrelated file changes.
• Python 3.12 / Django 5.2; full Python/Django matrix not run. The existing fork-in-multithreaded-process test emits a deprecation warning.
• Synthetic messages and mocked clients only. No hosted Postmark requests, actual email delivery, credentials, paid resources, or UI automation.

AI assistance: OpenAI Codex investigated, implemented, tested, and prepared this draft. Human verification is not claimed.