Why am I getting soft bounces and how do I fix them?
Updated on 1 Oct 2026: We added a diagnostic path for temporary DNS and connection failures and clarified how to read deferrals, codes, and retry outcomes.
Soft bounces are temporary delivery failures. A receiver can defer a message with a 4xx reply, or the sender can hit a temporary DNS or connection error before reaching the receiver. Common causes include full mailboxes, provider throttling after volume spikes or complaint increases, graylisting, temporary DNS failures, authentication errors, provider outages, and sender reputation problems. Message size and content policy limits can produce temporary or permanent failures, so use the complete delivery event to classify them.
The fix is not to guess. Start by reading raw SMTP replies or sender-side delivery errors, then group them by mailbox provider, failure reason, sending IP, campaign, template, and time window. A count like 5,000 soft bounces means very different things if the send volume was 5 million, 200,000, or 20,000. The percentage shows urgency. The failure detail shows cause.
- Export raw SMTP replies and sender-side errors rather than relying only on the sending platform's friendly category name.
- Separate recipient-side problems like full mailboxes from sender-side problems like throttling.
- Fix the specific cluster, slow the risky traffic, and only suppress addresses after a repeated mailbox-level pattern.
Why soft bounces happen
You are getting soft bounces because delivery has failed temporarily. The cause can be the recipient mailbox, the receiving provider, your sending infrastructure, your authentication, or your recent sending behavior. The exact reason is in the SMTP reply or the sending system's delivery log when no remote reply exists.
Treat soft bounces as a routing investigation. One cluster might be harmless, such as a few full mailboxes. Another cluster might be urgent, such as one provider deferring most traffic because the sending IP has low reputation. Another might be operational, such as one sending host being graylisted while the rest of the mail stream works.
|
|
|
|---|---|---|
Mailbox full | Recipient issue | Retry later |
Many at one provider | Provider deferral | Throttle sends |
One host | Graylisting | Fix retry |
Temporary auth error | Auth or DNS issue | Trace source |
Listed IP | Reputation | Pause source |
Large file | Size limit | Reduce email |
Fast mapping between soft bounce signals and practical fixes.
Soft bounce vs. hard bounce
A soft bounce is a temporary delivery failure, usually signaled by a 4xx SMTP reply. The sending system normally keeps the message queued and retries it. A dashboard can show the event as deferred before it becomes a final bounce, and a delay notice can arrive while retries continue.
|
|
|
|---|---|---|
SMTP class | 4xx | 5xx |
Meaning | Temporary failure | Permanent failure |
Queue action | Retry on schedule | Do not retry unchanged |
List action | Hold after repeated mailbox failures | Suppress invalid addresses; fix policy rejects |
The operational difference between temporary and permanent delivery failures.
Use the raw code and full response instead of assuming the dashboard label is exact. Sending platforms apply their own retry windows and can map provider-specific policy replies differently. A temporary deferral can later succeed, so track the final delivery outcome separately.
How to find the real cause
The sender dashboard category is only a starting point. Sending platforms often compress many different replies into one soft bounce label. That hides the useful detail. A full mailbox, a provider throttle, and a temporary authentication failure all need different fixes, but all three can appear as soft bounces in a campaign report.
A raw SMTP reply or delivery status notification can contain a basic SMTP status code and an enhanced status code. The basic code tells you whether the receiver treated the failure as temporary or permanent. The enhanced code adds context, such as mailbox storage, policy, authentication, or network trouble. A final delivery status notification might not appear until the sender's retry queue expires.
Example temporary delivery diagnosticstext
421 4.7.0 Try again later 451 4.7.1 Greylisted, please retry 452 4.2.2 Mailbox full 451 4.7.24 Temporary SPF evaluation error 451 4.4.2 Timeout while connecting to remote host
Work from evidence
A soft bounce investigation should start with the exact failure details recorded by the sending system. Without those details, every explanation is speculation.
- Export replies and local errors with the enhanced status code, recipient domain, sending IP, and timestamp.
- Build clusters by provider, campaign, source host, message type, and retry outcome.
- Change the traffic pattern that matches the evidence instead of changing everything.
SMTP soft bounce troubleshooting flowchart from raw reply to careful retry.
When there is no SMTP reply
A temporary lookup failure for a recipient domain's MX record or a connection timeout can stop delivery before the receiving server sends an SMTP reply. The sending system then records a local error, which might appear in the same soft bounce or deferred category as a remote 4xx response.
- Check the delivery log for the DNS result, destination host, connection error, and attempt time.
- Compare failures across recipient domains and sending hosts to isolate a local resolver or network problem.
- Let the queue retry temporary lookup and connection failures. Treat a confirmed nonexistent recipient domain as a permanent address problem.
Keep local errors separate from remote policy replies in reports. A provider cannot give a rate-limit instruction if the connection never reached it.
Soft bounce codes to check first
The first digit matters, but the complete response matters more. A 4xx reply signals a temporary failure that the sender should retry. A 5xx reply normally means permanent failure, but some sending platforms still place certain policy replies into a soft bounce bucket when they retry internally. Keep the code, enhanced status, and response text together.
|
|
|
|---|---|---|
421 | Service unavailable | Retry after a delay |
450 | Mailbox temporarily unavailable | Read full reply |
451 | Local processing error | Read full reply |
452 | Insufficient storage or limits | Check enhanced code |
451 4.7.24 | SPF evaluation error | Check DNS and retry |
Common reply families seen in soft bounce investigations.
When the code and text point in different directions, use the reply class, enhanced code, and full provider text together. If a reply says rate limited, slow that provider segment. If it says mailbox full, do not treat it like a global sender problem. If it names authentication, check SPF, DKIM, DMARC, and the return-path domain used by that stream. Do not assume every size failure is temporary because an X.2.3 message-length status is intended as a permanent failure.
Bounce fields to exporttext
recipient_domain smtp_code enhanced_status raw_smtp_reply sending_ip sending_host mail_from_domain header_from_domain campaign_id template_id first_attempt_time last_attempt_time retry_count
How to separate mailbox problems from sender problems
Recipient-side soft bounces tend to be scattered across many providers and campaigns. Sender-side soft bounces tend to cluster around one mailbox provider, one IP, one message stream, one campaign, or one time window. That split keeps the fix focused. An out-of-office auto-reply follows accepted mail and is not a soft bounce.
This also protects good subscribers. If the problem is a temporary provider throttle, suppressing every affected address punishes valid recipients. If the problem is repeated full mailboxes over several campaigns, a temporary hold makes sense.
Recipient-side pattern
- Mailbox full: The address exists, but the inbox has no room at the time of delivery.
- Temporarily disabled mailbox: The mailbox exists but is not accepting mail at the time of delivery.
- Old addresses: Aging lists produce more full or intermittently reachable mailboxes.
Sender-side pattern
- Provider throttling: One provider defers traffic when volume, complaints, or reputation look risky.
- Graylisted host: One sending host is told to retry later and fails if retry behavior is poor.
- Auth mismatch: A source sends with broken SPF, DKIM, DMARC, or return-path setup.
Authentication and reputation checks
After grouping the bounces, check whether the domain or sending source changed. A domain health check is useful when the bounce text points at DNS, authentication, or mixed sender setup. Some providers issue temporary deferrals for authentication or sender-requirement failures, while others reject the message permanently.
Suped's product brings DMARC monitoring, SPF, DKIM, hosted SPF, hosted DMARC, hosted MTA-STS, and real-time alerts into one workflow. Use Suped to connect a bounce cluster to authentication changes and follow source-specific fix steps instead of changing every DNS record.
Issue steps to fix dialog showing the issue overview, tailored fix steps, and verification action
Reputation checks matter too. If the replies mention policy, abuse, spam, denied connections, or listed infrastructure, check the sending IP and domain against blocklist (blacklist) data. Suped's blocklist monitoring helps identify listings tied to the affected sending source.
Do not fix DNS blindly
Only change authentication records after you connect the bounce cluster to the failing source. A global SPF or DMARC edit can affect working streams.
- Identify the exact vendor, host, and envelope domain tied to the failing replies.
- Compare SPF, DKIM, and DMARC pass rates before and during the bounce spike.
- Fix the failing stream first, then watch the next retry window for recovery.
Use a live test before resending
Before resending to a soft-bounced segment, send a fresh test through the same route when possible. Use the same envelope domain, header-from domain, DKIM selector, sending IP pool, and message template. A clean test does not prove every provider accepts the next campaign, but it catches obvious authentication and formatting problems.
A practical email tester result should show whether SPF, DKIM, DMARC, message headers, and content checks look sound before you put volume behind the stream again.
Email tester
Send a real email to this address. Suped shows a results button when the test is ready.
?/43tests passed
If the test shows authentication failures, fix those first. If authentication passes and the bounce cluster is provider-specific, investigate provider throttling or reputation. If the issue only appears with one template, reduce risky content, remove oversized attachments, and retest.
- Test using the same sending source that produced the soft bounce cluster.
- Keep the same header-from domain, return-path domain, and DKIM selector.
- Test the actual template if content filtering or size limits appear in replies.
- Start with one message, then use a small batch only after the route accepts it.
When graylisting is the answer
Graylisting is a temporary refusal that asks the sending server to try again later. The receiving system uses the retry to distinguish mail transfer systems that retry correctly from sources that give up or repeat too aggressively. When one sending host or IP produces many soft bounces and the rest of the program looks normal, graylisting belongs near the top of the list.
The key sign is concentration. If the failures all came from one host, one IP pool, or one automation process, the recipient list is probably not the root cause. The fix is to correct retry handling, adjust the sending pattern, and check whether the receiving provider sees the host as new or risky.
Graylisting style replytext
451 4.7.1 Greylisted, please try again later 451 4.7.1 Please retry after 900 seconds
Do not overreact to one graylisted host
A single graylisted source can create a large soft bounce count without meaning the whole domain has failed. Isolate the host before changing list rules.
- Confirm the sender retries after the requested delay instead of giving up.
- Reduce volume from that host until the receiving provider accepts traffic again.
- Track accepted, deferred, and expired retries separately for the same source.
Fixes by cause
Once the soft bounces are grouped, the fix is usually straightforward. The wrong move is applying one global response to every soft bounce. Full mailboxes need patient retry and later suppression. Provider throttling needs lower rate and better segmentation. Authentication failures need source-specific DNS fixes. Graylisting needs reliable retry behavior.
Soft bounce rate triage
Compare the current rate with its recent baseline, then inspect the largest provider and source clusters.
Normal watch
At baseline
Scattered temporary failures within the recent pattern
Investigate
Above baseline
Visible concentration or a material rate increase
Act now
Sharp spike
Large cluster, repeated deferrals, or authentication errors
- Full mailbox: Let the normal queue retry, then pause the address after repeated campaigns show the same issue.
- Provider throttle: Cut volume to that provider, send to recent engagers first, and extend spacing between batches.
- Auth failure: Fix the failing source, confirm SPF and DKIM pass, then watch DMARC results for that stream.
- Graylisting: Honor retry delays, keep the same sender identity on retry, and avoid sudden volume jumps.
- Listed source: Pause the affected IP or domain stream, remove risky recipients, and verify blocklist (blacklist) status.
For repeated temporary failures, use suppression logic that considers count, time, provider, engagement, and bounce reason. One temporary failure should not remove a good subscriber. Pause an address for repeated mailbox failures, but do not suppress recipients just because a provider deferred a stream.
A practical recovery plan
The fastest recovery plan is narrow and measured. Do not resend the entire failed audience immediately. Do not suppress the entire failed audience immediately. Identify the biggest cluster, fix that cluster, then retry a small sample.
A staged retry gives receiving providers time to accept normal traffic again without adding another spike. It also shows whether the change worked. If a small retry still defers, the root cause remains.
- Measure rate: Divide recipients with a temporary failure by recipients attempted in the same send and time window; report eventual deliveries separately.
- Find cluster: Rank by provider, code, source host, sending IP, campaign, and template.
- Fix source: Change the exact sending pattern, authentication setup, or retry behavior at fault.
- Retry sample: Let queued messages retry; after final failures, test with a small engaged segment once the cause is fixed.
- Update rules: Adjust address suppression only after repeated mailbox failures show that the address remains unreachable.
If the bounce spike came from a new list source, treat acquisition quality as part of the investigation. Old, scraped, unconfirmed, or poorly engaged addresses can cause delivery problems. A valid address can still be low quality for the mail stream if the owner does not engage or submits complaints. Prevent repeat clusters with confirmed opt-in, address checks at signup, an easy unsubscribe, and complaint monitoring.
If the spike came after a content change, compare the failing template against recent accepted templates. Attachments, URL density, URL reputation, image-heavy layouts, and sudden topic changes can increase filtering. Keep that review separate from DNS unless the bounce text names authentication.
Views from the trenches
Best practices
Group soft bounces by provider and SMTP reply before changing resend or hold rules.
Pull the raw bounce text because ESP categories often hide the condition that matters.
Compare bounce percentage against total send volume, not against the absolute count alone.
Check list age, engagement, and acquisition source before blaming the receiving ISP.
Common pitfalls
Treating every 4xx reply as harmless lets repeated temporary failures hurt reputation.
Resending all soft bounces at once can intensify throttling from the same provider.
Ignoring one noisy sending host can hide a graylisting issue that is easy to isolate.
Suppressing after one soft bounce removes valid recipients with temporary mailbox issues.
Expert tips
Keep a provider-level bounce dashboard so Gmail, Yahoo, Microsoft, and Apple stand out.
Store the sending IP, campaign, template, and bot or host name with every bounce event.
Use DMARC reports next to bounce data to separate authentication drift from mailboxes.
Set retry windows by bounce class because full mailboxes and throttling differ clearly.
Marketer view
Marketer from Email Geeks says the first step is to group soft bounces by classification and the receiving provider that generated them.
2023-05-15 - Email Geeks
Marketer view
Marketer from Email Geeks says the actual SMTP bounce text is key evidence when a receiving provider returns a temporary failure.
2023-05-15 - Email Geeks
