# Explaining a bounced email to a customer in plain words

> A customer forwards a bounce message and asks what it means. Learn to read the error, explain it without jargon, and give clear next steps for common causes.

- Published: 2026-10-17
- Author: Koltrix Team
- Category: Customer support
- Source: https://koltrix.com/blog/explaining-a-bounced-email-to-a-customer-in-plain-words

A customer writes: "I got this strange message and my email did not arrive. What does it mean?" Below it sits a block of text full of codes, server names and phrases like "550 5.1.1 user unknown". To them it looks like a failure of your product. To you it is a clear sentence in a technical dialect.

Your job is to translate. This guide shows how to read a bounce message quickly, explain each common cause in plain words, and tell the customer exactly what to do next.

## What a bounce message is

When an email cannot be delivered, a mail server sends back a notice. People call it a bounce, a "delivery failure" or "undeliverable" message. It comes from a mail system, not from a person, so the wording is stiff and the useful part is often hidden in the middle.

Every bounce has two things worth finding:

- **A status code**, such as 550 or 452, often with a second form like 5.1.1.
- **A reason in words**, such as "mailbox full" or "domain not found".

The first digit of the code tells you the big picture:

| Code starts with | Meaning | What it usually means for the customer |
| --- | --- | --- |
| 4 | Temporary problem | The server may still retry. Wait, or try again later. |
| 5 | Permanent problem | This message will not be delivered as it is. Something must change. |

If you want the full picture of the two types, [hard versus soft bounces](/blog/hard-vs-soft-bounces) explains them for senders.

## How to read one in 30 seconds

1. Find the line with the code and the words. Ignore the rest at first.
2. Look for the address it failed on. Is it spelled correctly?
3. Check which side the problem is on: the address that was typed, the receiving server, or the sending side.
4. Match it to one of the cases below.

## The common cases, in plain words

### "User unknown" or "mailbox does not exist" (5.1.1)

**What it means:** The address is wrong, or the person no longer has that mailbox.

**What to say:**

> The message was returned because the address it was sent to does not exist. It looks like there may be a typo in "jon.smith@exmaple.com". Could you check the spelling, or confirm the right address with the person?

Typos in the domain are the most common cause, so point at the exact characters.

### "Mailbox full" or "over quota" (4.2.2 or 5.2.2)

**What it means:** The recipient's inbox has no space left.

**What to say:**

> The recipient's mailbox is full, so it cannot take new mail. This is on their side. Please ask them to free up space, or contact them another way for now. You can try sending again once they have cleared it.

### "Domain not found" (5.1.2)

**What it means:** Nothing exists at the part after the @ sign. Often a typo, sometimes a domain that has expired.

**What to say:**

> The part after the @ sign is not recognized, so the message had nowhere to go. Please check "exmaple.com" against the website of the person you are writing to.

### "Message too large" (5.3.4)

**What it means:** The attachments make the email bigger than the receiver accepts.

**What to say:**

> The message was larger than the recipient's server allows. Try a smaller file, or share it with a download link instead. Our guide on [why attachments over 25 MB fail](/blog/why-email-attachments-over-25-mb-fail-and-what-to-send-instead) has more options.

### "Blocked as spam" or "rejected by policy" (5.7.x)

**What it means:** The receiving server did not like the message. It may have looked like spam, or the sender may be missing proper authentication.

**What to say, if the customer was the sender:**

> The recipient's mail system rejected the message as suspicious. This sometimes happens with certain words, many links, or attachments. Try sending a short plain message first, and ask the recipient to check their spam folder and to add you as a contact.

**If your own message to the customer was rejected,** this is your problem to investigate, not theirs. Check your sender setup before blaming their server. See [tracing delivery through the Received headers](/blog/tracing-delivery-received-headers).

### "Temporarily deferred" or "try again later" (4.x.x)

**What it means:** The receiving server is busy or has a short-term issue. The sending system keeps trying for a while.

**What to say:**

> This notice says the delivery is delayed, not failed. The system will keep trying for a day or two. You do not need to send it again unless you get a final failure notice.

Telling people not to resend avoids duplicates.

## Who is at fault, and how to say so

Be honest about whose side the problem is on:

- **Their typo or their recipient's mailbox:** say so kindly, with the exact text to check.
- **The recipient's server:** say it is outside your control, and offer another channel.
- **Your side:** say so first, say what you are doing, and say when you will update them.

Avoid "it is not our problem". Even if you cannot fix it, you can still help them reach the person.

## Write a reply template

Keep one short template for the common case:

> Hi [name], thanks for sending the message. The notice means [one plain sentence]. The part to check is [exact text]. Please [one clear step]. If it does not work, send me the new notice and I will take another look.

Keep technical terms out, or put them in one short line at the end for people who want them.

## What to ask for

If you need more, ask for:

- The full bounce message, copied as text, not a screenshot of part of it.
- The address it was sent to, and roughly when.
- Whether it happens with one recipient or all of them.

One recipient suggests a problem on their side. All recipients suggests something about the sender.

## What to remember

A bounce message is a precise answer wrapped in technical language. Find the code and the plain reason, match it to a common case, and tell the customer in one or two sentences what happened and what to do. Be clear whose side the problem is on, and own it when it is yours.