# How to read an email bounce message > When an email fails to deliver, the bounce message tells you exactly why - here is how to read one. Source: https://www.kualo.com/knowledgebase/email-troubleshooting/how-to-read-an-email-bounce-message Updated: 2026-06-14 --- When an email cannot be delivered, the receiving server sends back an automated message explaining why. These are called bounce messages, or Non-Delivery Reports (NDRs). They can look intimidating, but almost all the useful information is in two places: a three-digit code and a short reason in plain English. ## Where to look in a bounce message A bounce arrives in your inbox, usually from "Mail Delivery Subsystem", "Mailer-Daemon", or similar. Inside, look for: - **A three-digit code** such as `550`, `421`, or `452`. The first digit matters most (see below). - **A reason string** - a short phrase from the receiving server such as "User unknown", "Mailbox full", or "Message rejected due to policy". - **The affected address** - which recipient the failure relates to, useful if you emailed several people. You do not need to understand the whole message. Find the code and the reason, and the sections below will tell you what they mean. ## Temporary versus permanent: the first digit The first digit of the code tells you whether the problem is temporary or permanent, which is the single most important thing to know. - **A code starting with 4 (a 4xx code) is temporary.** The receiving server could not accept the message right now but the issue may clear. Your mail server will keep retrying automatically for a period (typically a couple of days) before giving up. Usually you do not need to do anything. - **A code starting with 5 (a 5xx code) is permanent.** The receiving server has refused the message and will not accept it on retry. These are the bounces that need your attention. So a `452` is something to wait on; a `550` is something to act on. ## Common bounce reasons and what to do ### The address does not exist Codes like `550 No such user`, `550 User unknown`, or `550 5.1.1` mean the recipient's address is not valid - it may be mistyped, or the account may have been closed. **What to do:** check the address for typos and correct it. If the address is definitely right but still bounces, the recipient's account may have been deleted, in which case you will need an alternative way to reach them. ### The recipient's mailbox is full A code such as `452 Mailbox full` or `422` means the recipient's inbox has hit its [storage limit](/knowledgebase/email-basics/email-storage-quotas-what-they-are-and-how-to-manage-them). This is temporary - your mail server will retry, and delivery often succeeds once they clear space. **What to do:** usually nothing, but if it is urgent you may want to contact the recipient another way to let them know. ### The message was blocked or rejected by policy Codes like `550 Message rejected`, `554`, or `552 Blocked by filters` - often with a reason mentioning spam, policy, or a blocklist - mean the receiving server actively refused your message. This category is the most worth understanding, because it points at your own sending setup rather than the recipient. Common causes are missing or incorrect SPF or DKIM authentication, or your sending IP or domain appearing on a blocklist. Our guide on [diagnosing outbound email delivery problems](/knowledgebase/email-troubleshooting/troubleshooting-outbound-email-delivery-problems-bounces-rejections-blacklists) walks through fixing each of these. :::info If the bounce names our server's IP address specifically, that is something we handle on your behalf. Contact our support team with the full bounce message and we will look into it. ::: ### A temporary deferral Codes like `421 Service not available` or `451 Try again later` mean the receiving server has asked your mail server to try again shortly. This is normal and common, often caused by greylisting (a deliberate anti-spam delay) or brief load on the receiving server. **What to do:** nothing. Your mail server retries automatically, and these almost always deliver on a later attempt. ### Still trying (a warning, not a failure) You may receive a message that is not a bounce at all but a progress notice, such as: ``` Warning: message still undelivered after 4 hours. Will keep trying until message is 2 days old. ``` This means delivery has not yet succeeded but your mail server is still attempting it. No action is needed. If it ultimately fails, you will receive a separate, final bounce. ## What about auto-replies and challenges? Not every automated message is a bounce: - **Out-of-office and auto-replies** confirm your message was delivered; the recipient is simply away. No action is needed, though if your message is time-sensitive, be aware they may not see it until they return. - **Challenge messages** come from recipients using a filter that only accepts mail from known senders. It will ask you to click a link or confirm you are a real person before your original message is released to them. Complete the step it describes if you want the message delivered. ## When to contact us Most bounces are either self-resolving or point at the recipient's side. Contact our support team if: - A bounce names **our server's IP address** as being on a [blocklist](/knowledgebase/senders-deliverability/what-to-do-if-your-domain-is-blacklisted). - You are receiving permanent (5xx) rejections you cannot explain after checking your authentication and the recipient address. When you get in touch, include the full bounce message and all its headers so we can see the rejecting server and the exact reason. You can [raise a support ticket](/knowledgebase/getting-started/how-to-create-a-support-ticket-in-mykualo) through the Kualo Client Area at any time. --- _Source: Kualo Knowledgebase — https://www.kualo.com/knowledgebase/email-troubleshooting/how-to-read-an-email-bounce-message · © Kualo Ltd._