---
title: "Delivery status and bounce handling"
description: "Classify SMTP and delivery-status failures, stop permanent retries, back off temporary failures, and connect outcomes to the correct recipient."
canonical: https://specification.email/spec/lifecycle/bounce-handling/
category: lifecycle
status: required
sources:
  - title: "RFC 3463 — Enhanced Mail System Status Codes"
    url: "https://www.rfc-editor.org/rfc/rfc3463"
    publisher: "IETF"
  - title: "RFC 3464 — Delivery Status Notifications"
    url: "https://www.rfc-editor.org/rfc/rfc3464"
    publisher: "IETF"
---

# Delivery status and bounce handling

> Classify SMTP and delivery-status failures, stop permanent retries, back off temporary failures, and connect outcomes to the correct recipient.

## What it is

Bounces and delivery-status notifications describe failed or delayed recipients with SMTP replies and structured enhanced status codes.

## Why it matters

Permanent failures need suppression; temporary failures need bounded retry. Treating both alike either wastes capacity or drops recoverable mail.

## How to implement

Parse machine-readable status data, preserve the original recipient correlation, distinguish 4.x.x from 5.x.x outcomes, and apply policy by failure class rather than free-text wording.

## Common mistakes

- Matching only English diagnostic phrases.
- Suppressing an address after one ambiguous timeout.
- Sending bounces to an unverified visible From address.

## Verification

Replay fixtures for user unknown, mailbox full, policy block, DNS failure, timeout, and delayed delivery. Each must follow the intended retry or suppression path.