Email bounce codes and rules
Learn why emails bounce, how MyLINK MarketingPlatform classifies bounce responses, and what the different bounce codes mean.
Before you start
It is useful to understand:
- The difference between hard, soft, and other bounces
- That bounce information comes from the recipient's mail server
- That different mail servers can return different or incomplete responses
- That some bounce reasons relate to the recipient address, while others relate to delivery, authentication, content, or sender reputation
Understand email bounces
When you send an email campaign, the recipient's mail server responds to the sending mail server.
If the message cannot be delivered normally, the response can be processed as a bounce.
MyLINK MarketingPlatform uses bounce codes to indicate why delivery failed or what type of response was received.
The exact reason is not always available. Receiving mail servers can return different responses for similar delivery problems, and some responses may not clearly identify the underlying issue.
The three types of email bounces
Email bounce responses are divided into three main types:
- Hard
- Soft
- Other
The bounce type helps you understand whether the issue is likely related to the recipient address, a temporary delivery problem, or another delivery condition.
| Bounce type | What it usually means | Is the address necessarily invalid? | What to investigate |
|---|---|---|---|
| Hard | A permanent or recipient-related condition | Often | Recipient address or recipient status |
| Soft | A temporary delivery condition | No | Mailbox, DNS, domain, or connectivity |
| Other | Delivery failed or another response was received for a different reason | Usually not | Content, reputation, authentication, server policies, or delivery processing |
Email bounce codes
Use the bounce code to understand how a failed or unusual delivery response was classified.
| Code | Description | Type | What it means |
|---|---|---|---|
| 0 | Non-bounce | Other | The bounce processor received an email that it determined was not a bounce notification. |
| 10 | Bad email address | Hard | The recipient address is invalid or the user does not exist. An example response is 550 User Unknown. |
| 20 | Temp failure - General | Soft | A temporary problem prevented delivery. The recipient server may ask the sender to try again later. |
| 21 | Temp failure - DNS Failure | Soft | A domain, host, or DNS-related issue prevented delivery. Responses can also include disabled-account conditions, so this code does not always mean that the domain itself is invalid. |
| 22 | Temp failure - Mailbox full | Soft | The recipient mailbox is full or has exceeded its quota. |
| 23 | Temp failure - Message size too large | Other | The message exceeds a size limit configured by the recipient server or its policies. |
| 29 | Temp failure - Unable to connect | Soft | The sending server could not establish a connection with the recipient's mail server. This can be caused by DNS issues, temporary availability problems, connection blocking, or in some cases an unknown or deactivated corporate address. |
| 30 | Bounce with no email address | Other | A bounce was detected, but the original recipient email address could not be identified. |
| 40 | General bounce | Other | The bounce could not be classified into a more specific category. |
| 50 | Mail block - General | Other | The recipient server rejected the message for a blocking reason that does not match a more specific mail-block category. |
| 51 | Mail block - Known Spammer | Other | The recipient server considers the sender to be a spammer. This can be related to public or private block lists. |
| 52 | Mail block - Spam detected | Other | The recipient server identified the message as spam or detected something associated with poor reputation, such as the content, reply-to address, or brand name. |
| 53 | Mail block - Attachment detected | Other | The recipient server rejected the message because it contains one or more attachments. |
| 54 | Mail block - Relay denied | Other | The recipient server refused to act as a relay for delivery of the message. |
| 55 | Mail block - SPF/DKIM/DMARC | Other | Delivery was rejected because of SPF, DKIM, or DMARC policies. |
| 59 | Mail block - Unable to connect | Other | The sending system could not connect to a large or known domain and considers blocking more likely than the domain having disappeared. |
| 60 | Auto-reply | Other | An automatic response was received, such as an out-of-office or vacation reply. |
| 70 | Transient bounce | Other | Delivery has been temporarily delayed. The remote server may continue trying to deliver the message for a limited period. |
| 90 | Unsubscribe request | Hard | An unsubscribe request was received through the bounce-processing system. |
| 100 | Challenge response | Other | The recipient server or service issued a challenge-response request. |
| 110 | Over throttle limit | Other | The message remained in the queue until its queue lifetime expired because another attempt would have exceeded configured delivery throttling. |
| 120 | Queue dumped | Other | Messages were removed from the delivery queue and bounced. |
| 121 | Delivery paused then message expired | Other | Delivery was paused and the message reached the maximum retry time before it could be delivered. |
| 200 | Spam complaint | N/A | A spam complaint was received. This is recorded separately from the standard Hard, Soft, and Other bounce categories. |
Hard bounces
A hard bounce generally indicates a permanent or recipient-related condition. A common example is an email address that does not exist.
Why hard bounces happen
Hard bounces can happen when:
- The recipient email address is invalid
- The recipient account does not exist
- An unsubscribe request is returned through the bounce-processing system
What to do
Review hard-bounced recipients before attempting another send. For an invalid recipient address, correct or remove the address from future sending. An unsubscribe request is different from an invalid address, even though code 90 is classified as a Hard bounce by the bounce processor.
Soft bounces
A soft bounce indicates a delivery problem that may be temporary. The email address itself may still be valid.
Why soft bounces happen
Soft bounces can happen when:
- The recipient mail server has a temporary problem
- DNS lookup fails
- A domain is temporarily unavailable
- The recipient mailbox is full
- The sending server cannot connect to the recipient's mail server
- The recipient server temporarily refuses the connection
A repeated soft bounce can eventually indicate a persistent problem. For example, a DNS failure may initially be temporary. If it continues, it may indicate that the domain no longer exists or is no longer configured correctly.
What to do
Do not assume that a single soft bounce means the email address is invalid. Review repeated soft bounces to determine whether the delivery issue appears temporary or persistent. The available bounce-code information does not specify a numeric soft-bounce threshold.
Other bounces
An Other bounce means that the response does not indicate a normal hard or soft recipient failure. The recipient address itself may still be valid.
Why Other bounces happen
Other bounces can happen when:
- The recipient server blocks the sender
- The sender is considered a known spammer
- Spam-like content is detected
- An attachment is rejected
- SPF, DKIM, or DMARC checks fail
- A relay request is denied
- The message is too large
- Connections from the sending server are blocked
- Delivery is delayed, paused, or expires in a queue
- An automatic reply is received
- A challenge-response system responds
What to do
Do not automatically treat the recipient address as invalid. Review the bounce code first.
Depending on the code, investigate:
- Email content
- Sender reputation
- Sender authentication
- Message size
- Recipient-server policies
- Delivery infrastructure
Special bounce responses
Some codes need extra explanation because they do not represent a standard delivery failure.
Code 0 - Non-bounce
The bounce processor received a message but determined that it was not a bounce notification.
Code 60 - Auto-reply
The response was generated automatically, for example by an out-of-office system. This does not by itself indicate that the recipient email address is invalid.
Code 90 - Unsubscribe request
Code 90 is classified as a Hard bounce, but an unsubscribe request is not the same as an invalid email address. It indicates that the recipient does not want to receive further messages.
Code 100 - Challenge response
The recipient system requires an additional challenge or verification step before accepting the message. The address itself may still be valid.
Code 200 - Spam complaint
A spam complaint means that the recipient reported the message as spam. This is recorded separately from the standard Hard, Soft, and Other bounce categories.
similar bounce codes
Some bounce codes describe similar situations but are classified differently.
Code 29 and code 59
Both codes can relate to connection problems.
Code 29 - Temp failure - Unable to connect
This is classified as a Soft bounce.
It can indicate:
- Temporary server unavailability
- DNS problems
- A domain problem
- Connection blocking
- In some cases, an unknown or deactivated corporate address
Code 59 - Mail block - Unable to connect
This is classified as Other. It is used when the destination is a large or known domain and the sending system considers blocking more likely than the domain having stopped existing.
Code 21 - Temp failure - DNS Failure
Code 21 is classified as a Soft bounce.
The returned response can indicate conditions such as:
- Domain or host resolution failure
- A host that cannot be found
- A DNS-related delivery problem
- A disabled account
This means that code 21 should not automatically be interpreted as proof that the recipient domain no longer exists. If the same condition continues, investigate whether the domain or recipient address remains valid.
When to use this information
Use bounce codes when reviewing email delivery results and investigating why messages were not delivered.
Common use cases
- Clean up recipient data
Use hard-bounce information to identify addresses that may no longer be deliverable. - Investigate temporary delivery problems
Use soft-bounce codes to distinguish temporary mailbox, DNS, and connection problems from permanent failures. - Troubleshoot sender reputation
Review mail-block codes when recipient servers reject messages because of spam or reputation concerns. - Troubleshoot email authentication
Use authentication-related bounce codes to identify SPF, DKIM, or DMARC delivery failures. - Troubleshoot message content
Review codes related to spam detection, attachments, and message size. - Investigate delivery infrastructure
Use connection, throttling, and queue-related codes when the recipient address itself does not appear to be the problem.
Marketing tip
Monitor the types of bounces you receive, not only the total number of bounces. A rise in invalid-address bounces can indicate problems with recipient data. A rise in mail-block, spam, or authentication bounces can indicate a deliverability, reputation, or configuration issue.
Additional information
SMTP responses
Bounce classifications are based on responses received during or after email delivery.
Different recipient mail servers can return different responses for similar problems, so a bounce response may not always identify the exact underlying cause.
Repeated soft bounces
A soft bounce can represent a temporary delivery problem.
If the same condition continues, the recipient address or domain may eventually need to be treated as invalid.
Mail blocks
A mail block does not automatically mean that the recipient email address is invalid.
Review the reason for the block before changing or removing the recipient.
Authentication
SPF, DKIM, and DMARC policies can cause the receiving server to reject a message even when the recipient address is valid.
Bounce processing
Some responses processed by the bounce system, such as auto-replies and challenge responses, are not standard permanent delivery failures.
Things to know
- Email bounce responses are classified as Hard, Soft, or Other.
- Hard bounces generally indicate a permanent or recipient-related condition.
- Soft bounces generally indicate temporary delivery problems.
- Repeated soft bounces can indicate a persistent problem.
- Other bounces can relate to spam filtering, authentication, sender reputation, content, server policies, or delivery processing.
- An Other bounce does not automatically mean that the recipient address is invalid.
- Codes 0, 60, and 100 do not represent the same type of recipient failure as a normal hard bounce.
- Spam complaints are recorded separately as code 200.
- An unsubscribe request is different from an invalid email address, even though code 90 is classified as Hard.
- Bounce information depends on the response returned by the recipient's mail server and may not always identify the exact underlying issue.
Troubleshooting
| Problem | Possible cause | Solution |
|---|---|---|
| An address returns code 10 | The recipient address may not exist | Review and correct the recipient address |
| A recipient has a mailbox-full bounce | The recipient has exceeded their mailbox quota | Treat it as a temporary condition and review repeated failures |
| A domain repeatedly returns DNS failures | The domain may be unavailable, misconfigured, or no longer active | Review repeated failures and verify the recipient domain |
| Emails are rejected as spam | The message or sender reputation may trigger spam filtering | Review the content, sender identity, reply-to address, and reputation |
| Emails fail SPF, DKIM, or DMARC checks | Sender authentication does not meet the recipient's policy | Review the sender authentication configuration |
| An email is rejected because of its size | The recipient server applies a message-size limit | Reduce the message size |
| Many recipients at the same domain cannot be reached | The receiving server may be unavailable or blocking the sender | Review the relevant connection or mail-block bounce codes |
| You receive an auto-reply | The recipient has an automatic response enabled | No address correction is normally indicated by the auto-reply itself |
| You receive a spam complaint | The recipient reported the message as spam | Review recipient consent, campaign targeting, and deliverability practices |