Why am I getting soft bounces and how do I fix them?
Michael Ko
Co-founder & CEO, Suped
Published 27 Apr 2025
Updated 23 May 2026
10 min read
Summarize with
Soft bounces happen because the receiving mail server did not accept the message right now, but it did not declare the address permanently invalid. The most common causes are full mailboxes, throttling, graylisting, temporary DNS failures, authentication failures, message size limits, content filtering, provider outages, and sending reputation problems.
The fix is not to guess. I start by reading the raw bounce messages, grouping them by mailbox provider, SMTP code, 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 tells you urgency. The bounce text tells you cause.
- First check: Export raw SMTP replies rather than relying only on the ESP's friendly category name.
- Next split: Separate recipient-side problems like full mailboxes from sender-side problems like throttling.
- Then act: Fix the specific cluster, slow the risky traffic, and only suppress addresses after a repeated pattern.
The direct answer
You are getting soft bounces because a receiving system is temporarily refusing your email. That temporary refusal can be caused by the recipient mailbox, the receiving provider, your sending infrastructure, your authentication, or your recent sending behavior. The exact reason is inside the bounce response.
I 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 of your traffic because your IP has low reputation. Another might be operational, such as one sending host or bot being graylisted while the rest of the mail stream works.
|
|
|
|---|---|---|
Mailbox full | Recipient issue | Retry later |
Many at Yahoo | Provider deferral | Throttle sends |
One host | Graylisting | Fix retry |
SPF fail | Auth drift | Update DNS |
Listed IP | Reputation | Pause source |
Large file | Size limit | Reduce email |
Fast mapping between soft bounce signals and practical fixes.
How to find the real cause
The sender dashboard category is only a starting point. ESPs 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.
The raw bounce line usually has 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 gives more context, such as mailbox storage, policy, authentication, or network trouble.
Example soft bounce repliestext
421 4.7.0 Try again later 451 4.7.1 Greylisted, please retry 452 4.2.2 Mailbox full 454 4.7.5 Temporary authentication failure 451 4.4.2 Timeout while connecting to remote host
Work from evidence
A soft bounce investigation should start with the exact messages returned by the receiving system. Without those replies, every explanation is speculation.
- Export replies: Pull SMTP text, enhanced status code, recipient domain, sending IP, and timestamp.
- Group tightly: Build clusters by provider, campaign, source host, message type, and retry outcome.
- Fix narrowly: Change the traffic pattern that matches the evidence instead of changing everything.
Flowchart showing a soft bounce investigation from raw bounce text to careful retry.
Soft bounce codes I check first
The first digit matters, but the text matters more. A 4xx reply means the receiver expects the sender to retry. A 5xx reply normally means permanent failure, but some ESPs still place certain policy replies into a soft bounce bucket when they retry internally. That is why I keep both code and text.
|
|
|
|---|---|---|
421 | Service busy | Slow retry |
450 | Mailbox busy | Retry later |
451 | Local error | Read text |
452 | Storage full | Hold briefly |
454 | Auth issue | Check DNS |
Common reply families seen in soft bounce investigations.
When the code and text disagree, I trust the text enough to investigate the named condition. If a reply says rate limited, I slow down that provider segment. If it says mailbox full, I stop treating it like a global sender problem. If it names authentication, I check SPF, DKIM, DMARC, and the return-path domain used by that stream.
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.
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.
- Auto-reply systems: Some systems produce temporary replies that look noisy in bulk reporting.
- Old addresses: Aging lists produce more full, disabled, 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 the bounce grouping, I 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. SPF and DKIM failures create temporary deferrals at some providers before they turn into outright rejection.
This is where Suped's product is practical. Suped joins DMARC monitoring, SPF, DKIM, hosted SPF, hosted DMARC, hosted MTA-STS, and real-time alerts in one workflow. For most teams, Suped is the best overall DMARC platform because it turns authentication failures into specific fix steps instead of leaving the sender to inspect every DNS record manually.
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 catch listings and reputation changes before they become repeated delivery failures.
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.
- Map sources: Identify the exact vendor, host, and envelope domain tied to the failing replies.
- Check pass rates: Compare SPF, DKIM, and DMARC pass rates before and during the bounce spike.
- Confirm scope: 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 sane before you put volume behind the stream again.
Email tester
Send a real email to this address. Suped opens the report when the test is ready.
?/43tests passed
Preparing test address...
If the test shows authentication failures, fix those first. If authentication passes and the bounce cluster is provider-specific, treat it as a provider throttling or reputation problem. If the issue only appears with one template, reduce risky content, remove oversized attachments, and retest.
- Same route: Test using the same sending source that produced the soft bounce cluster.
- Same identity: Keep the same header-from domain, return-path domain, and DKIM selector.
- Same content: Test the actual template if content filtering or size limits appear in replies.
- Same timing: Retest near the next planned send window to catch rate and retry behavior.
When graylisting is the answer
Graylisting is a temporary refusal that asks the sending server to try again later. It is meant to filter low-quality senders that do not retry properly. When one sending bot, host, or IP produces a large number of 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 421 4.7.0 Temporarily deferred due to rate limits 451 4.7.1 Please retry after 900 seconds
Do not overreact to one graylisted host
A single graylisted source can create a scary soft bounce count without meaning the whole domain has failed. Isolate the host before changing list rules.
- Check retry: Confirm the sender retries after the requested delay instead of giving up.
- Warm slowly: Reduce volume from that host until the receiving provider accepts traffic again.
- Keep evidence: 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
Use percentage first, then inspect the largest provider and source clusters.
Normal watch
Under 1%
Scattered temporary failures across providers
Investigate
1% to 3%
Visible concentration by provider or source
Act now
Over 3%
Large cluster, repeated deferrals, or auth errors
- Full mailbox: Retry over a short window, 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, I use suppression logic that considers count, time, provider, engagement, and bounce reason. One temporary failure should not remove a good subscriber. A repeated pattern across multiple sends should stop mail until the address or provider recovers.
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.
I prefer a staged retry because it gives receiving providers a chance to accept normal traffic again without adding another spike. It also tells you whether the change worked. If a small retry still defers, the root cause remains.
- Measure rate: Divide soft bounces by delivered plus bounced attempts for the same send.
- 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: Start with engaged recipients and a small batch before restoring normal volume.
- Update rules: Adjust suppression only after repeated failures prove the address is no longer reachable.
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 cause soft bounces before they become hard bounces. A valid address can still be low quality for your mail stream if the owner never opens, complains, or ignores every campaign.
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 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 from Email Geeks says the actual SMTP bounce text is the only reliable source when a temporary failure spike needs an answer.
2023-05-15 - Email Geeks
The shortest path to a fix
Soft bounces are not a diagnosis. They are a signal that the receiving side refused delivery temporarily. The fix starts when you read the raw reply and find the pattern behind it.
If the failures are scattered, handle them with normal retry and suppression rules. If they cluster around one provider, source, host, or code, slow the affected traffic and fix that source. If authentication or reputation changed, treat it as an infrastructure issue before you resend.
Suped fits this workflow when the cause touches DMARC, SPF, DKIM, hosted SPF, blocklist (blacklist) monitoring, or cross-domain visibility. Bounce data tells you what failed at delivery time. Suped helps show whether the sending identity behind that failure is healthy enough to keep sending.
