Permanent failures do not retry

Retrying a hard bounce five times does not deliver the email — it generates five additional bounce events against your sending domain. Bounce rate is the primary metric email providers use to classify senders as spam sources. A sustained bounce rate above 2% triggers deliverability penalties from Gmail and Yahoo that affect all emails from the domain, including transactional messages to valid recipients. CWE-390 covers detection of errors without action; re-throwing permanent failures into the retry pipeline is the application-layer pattern that drives deliverability degradation.

Acknowledge permanent failures immediately in workers/email.worker.ts without re-throwing into the retry pipeline:

try {
  await esp.send(job.data)
} catch (err: unknown) {
  const kind = classifyEspError(err)
  if (kind === 'permanent') {
    await db.recipient.update({
      where: { email: job.data.to },
      data: { deliverabilityStatus: 'undeliverable', reason: String(err) }
    })
    logger.warn({ to: job.data.to }, 'Permanent failure — not retrying')
    return // Return without throwing — BullMQ marks job complete
  }
  throw err // Re-throw transient errors for BullMQ backoff
}

Distinguish at minimum: 400 (invalid address), 403 (blocked), and 422 (unsubscribed) as permanent failure codes that must not enter the retry path.