# Types of email bounces

> A guide to the different types of email bounce codes and what each one means for your message delivery.

Source: https://www.kualo.com/knowledgebase/email-troubleshooting/types-of-email-bounces
Updated: 2026-06-04

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When an email cannot be delivered, the sending server returns a bounce message with a code that describes why. Understanding these codes helps you diagnose and fix delivery problems.

## Bounce types

### Address change (`bounce_ac`)

The recipient has changed their email address and their old account is sending an automatic reply to notify senders of the new one. Update your records with the new address.

### Auto-reply (`bounce_ar`)

Auto-replies - typically out-of-office notifications - are sent by the recipient's email client. They do not mean delivery failed; they simply indicate the recipient is temporarily unavailable. This matters when you are sending time-sensitive information, as the recipient may not see your message until they return.

### Challenge/response (`bounce_cr`)

Some recipients use filtering software that only accepts mail from known senders. When your message arrives, the filter sends you a challenge - usually asking you to click a link or complete a simple action - to confirm you are a real sender. If you do not complete the challenge, your original message is not delivered.

### DNS failure (`bounce_df`)

The receiving server cannot deliver your message because of a temporary DNS problem. This is usually transient; the sending server will retry automatically. If it persists, the domain's DNS records may be misconfigured.

### Hard bounce (`bounce_hb`)

A hard bounce means the message was permanently rejected. Common causes are:

- The recipient's domain does not exist.
- The recipient's email address is unknown or has been deleted.

You should remove hard-bouncing addresses from your mailing list, as continuing to send to them can harm your sender reputation.

### Email blocked (`bounce_mb`)

The recipient's mail server is actively blocking email from your server. You may see error messages such as:

```
550 Message REFUSED by peer
552 Blocked by filters
```

This can happen if your server's IP address is on a blocklist. Check your IP reputation and, if necessary, contact your hosting provider for assistance.

### Mailbox full (`bounce_mf`)

The message could not be delivered because the recipient's mailbox has reached its storage limit. This is a temporary condition - delivery may succeed once the recipient frees up space.

### Soft bounce (`bounce_sb`)

A soft bounce means the message reached the recipient's mail server but was returned before final delivery. The most common cause is a full inbox, but it can also result from a temporary server issue. The sending server will retry delivery automatically. After several failed attempts, a soft bounce is typically reclassified as a hard bounce.

### Transient bounce (`bounce_tr`)

A transient bounce is generated by the sending server to let you know that a message has not yet been delivered but that the server is still trying. You may see a notice such as:

```
Warning: message still undelivered after 4 hours. Will keep trying until message is 2 days old.
```

In most cases you can safely ignore a transient bounce and wait for the server to retry. If the message is still undelivered after the retry window expires, it will become a hard bounce.


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_Source: Kualo Knowledgebase — https://www.kualo.com/knowledgebase/email-troubleshooting/types-of-email-bounces · © Kualo Ltd._