fix(smtp): prevent double STARTTLS causing "Connection already using TLS" error

[kyle-li-dev]

Summary

Fix SMTP STARTTLS double execution that causes EmailConnectionError: SMTP connection error: Connection already using TLS when using SMTPConfig(use_tls=True, use_ssl=False) with any STARTTLS-capable server (port 587).

Closes #19

Problem

When connecting to an SMTP server that supports STARTTLS (e.g., smtp.office365.com:587), the TLS upgrade is executed twice:

Step	Actor	What happens
1	litestar-email	Creates aiosmtplib.SMTP without passing start_tls (defaults to None)
2	aiosmtplib	start_tls=None → auto-detect → server supports STARTTLS → auto-upgrades ✅
3	litestar-email	use_tls=True → manually calls starttls() again → 💥 Connection already using TLS

This affects all SMTP servers requiring STARTTLS (Office 365, Gmail, AWS SES, etc.), making the documented use_tls=True + use_ssl=False configuration completely broken.

Solution

Explicitly pass start_tls=False to aiosmtplib.SMTP() to disable its auto-STARTTLS negotiation, letting litestar-email manage the STARTTLS upgrade manually as intended:

self._connection = aiosmtplib.SMTP(
    hostname=self._config.host,
    port=self._config.port,
    timeout=self._config.timeout,
    use_tls=self._config.use_ssl,
    start_tls=False,  # Disable auto-STARTTLS; we manage it manually below
)

This is the minimal, non-breaking fix (Option A from the issue). The existing manual starttls() call remains the single point of TLS upgrade.

Changes

• src/litestar_email/backends/smtp.py — Add start_tls=False parameter to aiosmtplib.SMTP() constructor
• src/tests/test_smtp_backend.py — Add 3 regression tests:
  • test_smtp_backend_starttls_no_double_call — Verifies start_tls=False is passed and manual starttls() is called exactly once
  • test_smtp_backend_ssl_does_not_call_starttls — Verifies implicit SSL (use_ssl=True) does not trigger manual STARTTLS
  • test_smtp_backend_no_tls_no_ssl — Verifies plain connection does not call STARTTLS

Checklist

• make test passes (129 tests)
• make lint passes (ruff, mypy, pyright, slotscheck)
• Regression tests added
• No breaking changes

[kyle-li-dev]

@cofin PTAL, thanks!

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[cofin]

Thanks @kyle-li-dev