Hard bounce results in immediate permanent suppression

SMTP 5xx codes (550, 551, 553, 554) are permanent rejections from the receiving server, per RFC5321-s3.6 — the address does not exist, the mailbox is closed, or delivery is permanently refused. Continuing to send to permanently invalid addresses does two things: it increases your hard bounce rate (ISPs track this per IP and per domain), and it confirms to ISPs that you are not maintaining list hygiene. Google and Yahoo both publish hard bounce rate thresholds; exceeding them triggers increasingly aggressive filtering that affects deliverable addresses in the same campaign. Suppression must be immediate, not batched.

Handle hard bounce events in your ESP webhook immediately — upsert a suppression record and check it on every subsequent send attempt. The suppression check must happen before queuing the message, not at delivery time:

export async function handleBounceEvent(event: BounceEvent): Promise<void> {
  const isPermanent =
    event.type === 'hard' ||
    event.bounceCode >= 550 ||
    event.reason?.toLowerCase().includes('does not exist')

  if (isPermanent) {
    await db.suppression.upsert({
      where: { email: event.email.toLowerCase() },
      update: { reason: 'hard_bounce', updatedAt: new Date() },
      create: {
        email: event.email.toLowerCase(),
        reason: 'hard_bounce',
        source: 'bounce_webhook',
        suppressedAt: new Date()
      }
    })
  }
}

export async function isAddressSuppressed(email: string): Promise<boolean> {
  return (await db.suppression.findUnique({
    where: { email: email.toLowerCase() }
  })) !== null
}

Call isAddressSuppressed as part of the pre-send check, before the job enters the queue. Cross-reference deliverability-engineering.bounce-fbl.soft-bounce-escalation — soft bounces also require escalation logic to eventually reach permanent suppression.