A lasting delivery problem
Usually an invalid or closed mailbox, or a destination that cannot receive your email.
Your next step
Suppress confirmed invalid addresses. Verify a correction before using it.
What is an email bounce?
A bounce is a delivery failure: the receiving mail system cannot accept or deliver your message. The response helps explain whether the problem concerns the address, the receiving server or the message you sent.
A temporary delay may still lead to successful delivery. Check the final campaign result before deciding what to do.
A temporary obstacle
Often a busy server, a temporary connection issue or a receiving service asking you to wait.
Your next step
Allow managed retries, then check the final result. Review recurring failures.
Three clues in a bounce response
The complete server response is more useful than a short category name. Here is an illustrative example:
550 5.1.1 Recipient address rejected: user unknownSMTP reply Enhanced status Diagnostic text
The SMTP reply
A 4xx reply normally asks the sender to try later. A 5xx reply rejects the current attempt permanently; repeating it unchanged is unlikely to help.
The enhanced status
A dotted code such as 5.1.1 adds detail. Here it identifies a bad destination mailbox address. The first digit describes the failure class.
The diagnostic text
Read the words after the code for the provider’s explanation. The same basic reply can cover very different problems.
Technical reference: IETF enhanced mail status codes ↗
20 Common reasons your email bounced
Find the closest reason below, then check it against your full response. These 20 examples cover address problems, temporary delays, authentication and policy blocks. They are not a frequency ranking or a universal code-to-category mapping.
421 / 4.7.xRate limited or temporarily deferredUsually temporary
Why this happens: Rate limited or temporarily deferred
The receiving service is limiting traffic or asking the sending server to wait. Reputation, volume or a temporary policy check may be involved.
Next steps: Rate limited or temporarily deferred
Allow the sending service to manage retries. If this repeats across a domain, ask support to review the response and sending pattern.
450 / 451Mailbox or server temporarily unavailableUsually temporary
Why this happens: Mailbox or server temporarily unavailable
The receiving system cannot complete delivery right now. Its diagnostic text may describe maintenance, a local processing error or a mailbox issue.
Next steps: Mailbox or server temporarily unavailable
Review the final delivery status after retries. Investigate repeated failures rather than manually sending the same campaign again.
4.2.2 / 5.2.2Mailbox fullCheck the response
Why this happens: Mailbox full
The recipient has reached a storage limit. Servers can report this with either a temporary or a permanent status.
Next steps: Mailbox full
Follow the actual response and your campaign’s final classification. If the problem keeps recurring, stop mailing the address and confirm it through another channel.
550 / 5.1.1Recipient address does not existAddress failure
Why this happens: Recipient address does not exist
The receiving service reports that the destination mailbox is unknown. A typing mistake, closed account or outdated contact record may be responsible.
Next steps: Recipient address does not exist
Suppress the invalid address. Only use a corrected address after confirming it with the subscriber; do not guess alternatives.
5.1.2Destination domain is invalidAddress failure
Why this happens: Destination domain is invalid
The domain after the @ sign is invalid for mail. A temporary DNS lookup failure is a different issue and needs its own diagnosis.
Next steps: Destination domain is invalid
Check for a data-entry error and confirm the correct address with the subscriber. Keep the failed address out of future sends.
5.7.1 / 5.7.26Policy block or authentication failureCheck the response
Why this happens: Policy block or authentication failure
The server rejected the message for a security, permission or authentication reason. This does not by itself mean that the recipient address is invalid.
Next steps: Policy block or authentication failure
Read the full diagnostic. Check sender authentication, permission and any provider-specific instructions before considering another send.
5.2.3 / 5.3.4Message exceeds a size limitCheck the response
Why this happens: Message exceeds a size limit
The message is larger than the receiving mailbox or system allows. The address itself may still be valid.
Next steps: Message exceeds a size limit
Reduce message size and attachments, using links to hosted files where appropriate. Resolve the cause before trying again.
4.4.xNetwork, routing or connection problemUsually temporary
Why this happens: Network, routing or connection problem
Delivery has been delayed by a connection or routing issue. If the retry window expires, the message can still end with a failed status.
Next steps: Network, routing or connection problem
Wait for the final result. Escalate persistent or widespread failures with the affected domain and complete diagnostic text.
5.1.3Recipient address has invalid syntaxAddress failure
Why this happens: Recipient address has invalid syntax
The email address is incorrectly formatted. Missing characters, extra spaces or a malformed address can prevent delivery.
Next steps: Recipient address has invalid syntax
Check the original signup record and confirm the correct address with the subscriber. Do not guess a replacement.
4.2.1 / 5.2.1Mailbox disabled or not accepting messagesCheck the response
Why this happens: Mailbox disabled or not accepting messages
The mailbox exists but is not accepting messages. It may be suspended, inactive or temporarily disabled.
Next steps: Mailbox disabled or not accepting messages
Use another channel to confirm whether the account is still used. Follow the response classification and stop sending to permanently closed accounts.
451 / 4.7.1Greylisting: first delivery attempt deferredUsually temporary
Why this happens: Greylisting: first delivery attempt deferred
Some receiving servers deliberately delay an unfamiliar sender. The response may specifically mention greylisting or ask the sending server to try again later.
Next steps: Greylisting: first delivery attempt deferred
Let the sending service retry automatically. If delivery never succeeds, ask support to check the complete response; not every 4.7.1 means greylisting.
5.7.1 / 5.7.0Message flagged as spam or unwanted contentCheck the response
Why this happens: Message flagged as spam or unwanted content
The diagnostic says the message was blocked by a spam filter or content policy. The same codes can also describe other security restrictions.
Next steps: Message flagged as spam or unwanted content
Review consent, message content, links and sending reputation with support. Fix the stated issue before sending again; avoid repeated attempts or filter-evasion tricks.
4.4.3Temporary DNS or directory lookup failureUsually temporary
Why this happens: Temporary DNS or directory lookup failure
The mail system could not obtain the information needed to route the message. A DNS lookup or directory service may be temporarily unavailable.
Next steps: Temporary DNS or directory lookup failure
Allow managed retries. If failures persist, share the affected domain and full error with support so the domain administrator can investigate.
4.4.7 / 5.4.7Delivery retry window expiredCheck the response
Why this happens: Delivery retry window expired
The message remained undeliverable until the sending system stopped trying. This is the final outcome of an earlier problem, not necessarily an invalid address.
Next steps: Delivery retry window expired
Look for the earlier connection, mailbox or policy error. Check the final campaign result and resolve that cause before considering a fresh send.
5.4.6Forwarding or routing loopCheck the response
Why this happens: Forwarding or routing loop
The message keeps being passed between mail systems instead of reaching the recipient. A forwarding rule or mail-routing configuration may be responsible.
Next steps: Forwarding or routing loop
Ask the recipient to have their mail administrator check forwarding and routing. Repeatedly resending the same message will not repair the loop.
5.7.23SPF authentication failedCheck the response
Why this happens: SPF authentication failed
The receiving server reports an SPF failure: the sending server is not authorised by the relevant sending domain’s SPF policy.
Next steps: SPF authentication failed
Ask support or your domain administrator to review SPF using the exact settings in Sender Profiles. Do not add a second SPF record.
5.7.20DKIM signature could not be verifiedCheck the response
Why this happens: DKIM signature could not be verified
The receiver reports that no DKIM signature passed verification. The signing setup, DNS key or a change to the signed message may need investigation.
Next steps: DKIM signature could not be verified
Give support the complete diagnostic. Check the DKIM records against Sender Profiles and investigate signing or message changes before trying another campaign.
5.7.25Sending server reverse DNS check failedCheck the response
Why this happens: Sending server reverse DNS check failed
The receiving service could not validate the sending IP address’s reverse DNS. The sending mail-server configuration needs checking.
Next steps: Sending server reverse DNS check failed
Contact the sending service with the full response. Reverse DNS is normally managed by the IP owner; changing the recipient address will not fix it.
550 / 554Message rejected: read the detailCheck the response
Why this happens: Message rejected: read the detail
These general replies cover several causes. A 550 is not always an unknown user; a 554 is not automatically a temporary error.
Next steps: Message rejected: read the detail
Use the enhanced status code and response text to distinguish an invalid address from a content, authentication or policy problem.
No clear codeUnspecified delivery failureCheck the response
Why this happens: Unspecified delivery failure
The receiving system supplied too little information to identify a reliable cause.
Next steps: Unspecified delivery failure
Check whether failures share a domain or campaign. Give support the full response rather than assuming that the address is invalid.
No matching reasons. Try a shorter code or a word such as “server”. If the response is unclear, our support team can help.
Provider detail: Google’s SMTP errors and codes ↗ · IANA enhanced status code registry ↗
A clear next step for every failure
Read the final result
Open your campaign analytics and review the bounce reason. A deferred attempt may later succeed; avoid sending a duplicate while delivery is still being retried.
Separate invalid addresses from blocked messages
Keep confirmed invalid recipients out of future campaigns. For authentication, content or policy blocks, investigate the sending issue rather than treating every recipient as a bad address.
Look for a pattern
Failures concentrated at one provider may indicate a provider-specific policy or rate limit. Failures across a newly imported list may point to stale records or data-entry problems.
Resolve the cause before the next send
Preserve suppression records and do not re-import bounced contacts to bypass them. If you need help, share the campaign, affected domain, time of send and complete error text with our team.
Reduce the bounces you can prevent
- Collect addresses carefully. Use clear signup forms and address confirmation to reduce mistakes and unwanted subscriptions.
- Keep contact records current. Review recurring failures and remove confirmed invalid addresses from future sends.
- Verify your sending domain. Check SPF, DKIM and DMARC with your domain administrator. Use the exact settings in your logged-in Sender Profiles.
- Send to an audience that expects you. Avoid purchased lists and sudden sends to old, unverified records.
- Keep messages manageable. Link to large files and check message size, sender details and destination links.
- Make leaving easy. Provide a clear unsubscribe route and respect it; complaints can contribute to reputation and delivery problems.
Gmail requires SPF or DKIM for all senders to personal Gmail accounts. Bulk senders must meet additional requirements, including SPF, DKIM, DMARC alignment and one-click unsubscribe for marketing messages. Authentication helps establish trust, but does not guarantee inbox placement.
Read Gmail’s current sender requirements ↗ · Our domain verification guide →
What does your bounce rate tell you?
Bounce rate measures the share of attempted deliveries that failed. Check your reporting tool’s definition and whether retries have finished before comparing campaigns.
Bounce rate = failed deliveries ÷ attempted deliveries × 100
Example: 20 final failures from 1,000 attempted deliveries = 2%.
A sudden increase deserves investigation, even if your overall rate looks low. Compare the list source, affected domains and reasons with earlier campaigns. There is no single percentage that guarantees a healthy list or successful delivery.
A message accepted by a server may still go to spam. “Delivered” does not prove inbox placement, and a bounce is different from an unsubscribe or a spam complaint.
Email bounce questions, answered
Should I delete every address that bounces?
No. Suppress confirmed invalid addresses, but investigate temporary failures and policy blocks first. Preserve suppression records so a later import does not accidentally put invalid contacts back into a campaign.
Can a soft bounce turn into a hard bounce?
Repeated temporary failures can eventually lead a platform to suppress an address. A full mailbox may have been abandoned, for example. The number of attempts and timing depend on the service and the reason; check the final result.
Does a 550 response always mean the address is invalid?
No. Read the enhanced status and diagnostic text. A 550 with 5.1.1 commonly indicates an unknown mailbox, while a 5.7.x response may describe a policy or authentication rejection.
Should I resend immediately after a bounce?
Allow automatic delivery retries to finish for temporary failures. For a permanent rejection, resolve the stated cause first. Repeated manual sends can create duplicates or worsen rate limiting.
Is an out-of-office reply a bounce?
Usually not. An automatic holiday reply generally means a mailbox received the message and sent a response. It does not, on its own, indicate failed delivery.
Will verifying my domain fix every bounce?
No. Authentication addresses one important group of failures, but cannot repair a closed mailbox, a misspelt recipient address or a receiving server outage.