What causes Microsoft S3150 blocklist bounces and how can I resolve them?
Updated on 17 Oct 2026: We clarified Microsoft's delisting routes and added steps for verifying recovery and resuming delivery after S3150 blocks.
Microsoft S3150 bounces happen when Microsoft rejects mail because the outbound IP or part of its sending network is on an internal blocklist. Resolve them by saving the full bounce, confirming the affected IPs and recipient domains, checking reputation and authentication, then using the Microsoft support route named in the NDR. If the first answer says no issue was found, follow the case's reply instructions and provide newer S3150 samples.
Treat S3150 as an IP reputation and Microsoft filtering problem first. Check authentication separately: verify SPF and DKIM, and confirm that DMARC passes for the visible From domain. A correct DNS setup does not automatically remove a Microsoft block.
- First move: Collect complete bounce text, sending IP, timestamp, sender domain, and recipient domain before changing anything.
- Main test: Check whether the failures are limited to Outlook, Hotmail, Live, MSN, and Microsoft 365 recipients.
- Support request: Fix obvious sending issues, then submit multiple fresh bounce examples through the route named in the NDR.
What S3150 means
The S3150 wording usually appears inside a 550 5.7.1 non-delivery report. The key phrase is that part of the sender's network is on Microsoft's block list. That points to an IP or network reputation decision inside Microsoft, not a mailbox typo, bad MX record, or generic SMTP outage. The wording does not reveal the size of the affected network, so do not assume that a full /24 or every IP at the provider is blocked.
Common S3150 bounce wording
550 5.7.1 Unfortunately, messages from [203.0.113.10] were not sent. Please contact your Internet service provider since part of their network is on our block list (S3150).
A Microsoft thread records a sender reporting the same rejection despite clean public blacklist checks. The report does not establish the size of the network Microsoft blocked.
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S3150 | Sending IP or network block | Use Outlook.com sender support for consumer recipients |
S3140 | Similar IP or network rejection wording | Read the full NDR and identify the rejected IP |
5.7.1 | General security or policy rejection | Read the diagnostic text; this code alone does not identify S3150 |
5.7.515 | Outlook.com authentication requirement failure | Correct SPF, DKIM, and DMARC |
5.7.606-649 | Microsoft 365 banned sending IP | Follow the NDR to sender.office.com |
4xx | Temporary deferral | Retry with backoff; reduce sending rate for rate-limit responses |
How to distinguish Microsoft SMTP rejection codes
Why Microsoft issues S3150 bounces
Sending IP reputation and authentication checks for Microsoft S3150 bounces
S3150 identifies an IP or network block but does not disclose the triggering signal. Investigate complaint spikes, spam trap hits, compromised traffic, shared-IP abuse, sudden volume changes, and old mail streams reactivated too quickly. Microsoft's sender troubleshooting guidance describes reputation factors, including complaints and list accuracy; the S3150 code alone does not diagnose which factor triggered a block.
S3150 can also begin across several Microsoft recipient domains while sender-side checks remain stable. Treat that pattern as a reason to collect affected IPs, recipient domains, timestamps, and fresh failures for escalation. It is not proof of a Microsoft outage.
- IP listing: Microsoft has classified the connecting IP as risky, blocked, or part of a risky network.
- Network range: The rejection can involve more than one outbound IP, so test each IP instead of inferring the range size from the wording.
- Volume jump: Check for sudden increases in volume or connection rate, particularly on a new or previously idle sending IP.
- Authentication and DNS: Check SPF, DKIM, DMARC, reverse DNS, and HELO/EHLO without assuming that an error in these checks caused S3150.
- Review needed: Request Outlook.com review when its S3150 bounces continue, even if the separate Microsoft 365 delist portal reports no block.
How to confirm the cause
Start by separating Microsoft-only failures from global delivery failures. If non-Microsoft providers accept the same mail while Outlook and Hotmail bounce S3150, the problem is centered on Microsoft reputation or routing. If many providers reject mail, the issue is broader than S3150. Use the public connecting IP shown in the NDR, which can belong to an outbound relay or NAT gateway rather than your web server or domain's inbound MX.
Then check whether the affected IP appears on public blocklists (blacklists). Microsoft can block an IP even when public checks look clean. Investigate any public listing under that list's own criteria; removal there does not automatically clear Microsoft's internal block.
Blocklist checker
Check your domain or IP against 144 blocklists.
Inspect authentication headers on a separate, successfully received test sent through the same outbound route. An SMTP-rejected message has no recipient-side delivered headers to inspect. Suped's email tester helps check authentication on the accepted test. For a wider DNS review, use Suped's domain health check before submitting the ticket.
Evidence to collect before escalation
Affected public outbound IP: 203.0.113.10 Sender domain: example.com Recipient domain: outlook.com UTC time: 2026-08-10 03:14 Receiving SMTP host: [copy from sending logs] SMTP response: [paste the complete S3150 response and diagnostic identifiers] Message type: transactional password reset Recent volume change: none Authentication on a separate accepted test: SPF pass, DKIM pass, DMARC pass
Choose the correct Microsoft delisting route
Use the recipient type, complete SMTP response, and instructions in the NDR to choose the route. S3150 commonly appears when sending to Microsoft's consumer domains, while Microsoft 365 has a separate banned-sending-IP flow with different enhanced status codes. Microsoft's delisting documentation distinguishes the consumer support route from Microsoft 365 blocked-IP requests.
- For S3150 affecting Outlook.com, Hotmail, Live, or MSN recipients, start with Outlook.com sender support.
- If the NDR directs you to sender.office.com, including Microsoft 365 banned-IP responses such as 5.7.606-649, use the Microsoft IP delist portal.
- For the Microsoft 365 portal, open the email confirmation link, then select 'Delist IP'. Confirmation alone does not complete removal.
- For a Microsoft 365 5.7.511 response, follow the NDR's manual review instructions, including forwarding the full NDR and affected IP to delist@microsoft.com. The self-service portal does not handle that code.
- If the Microsoft 365 portal reports no block but new Outlook.com S3150 bounces continue, use the consumer support route. Keep the case number and provide fresh samples through the case's stated follow-up channel.
Submit through the IP owner
The organization that controls the public outbound IP should submit the request. If the IP belongs to a shared relay, ISP, or sending provider, send that provider the full NDR and timestamps so its abuse or deliverability team can investigate the range and contact Microsoft. A dedicated IP supplied by a sending service is still managed by that service; ask it to coordinate the IP review while your IT team checks domain DNS.
How to resolve S3150 bounces
Microsoft S3150 resolution steps, including bounce collection and support escalation
The workflow is short, but the order matters. A ticket with vague wording often gets a generic response. Fixing DNS without asking Microsoft to review the rejected outbound IP can leave the block in place.
- Save evidence: Keep the raw bounce, not a screenshot or summary. Microsoft needs the exact SMTP response.
- Map scope: Group failures by sending IP, recipient domain, message stream, and first-seen time.
- Fix basics: Correct SPF, DKIM, DMARC, PTR, HELO, list hygiene, compromised accounts, and unsafe queue behavior.
- Retest once: After corrective action, send a new controlled message to a Microsoft recipient and record the result. Stop automatic retries of the rejected delivery after a 5xx response.
- Open the case: Use the route named in the NDR and submit the IP, UTC timestamps, bounce text, corrective action, and operational purpose of the mail.
- Reply again: If Microsoft says no issue exists, provide newer bounce samples from the affected IP using the existing case's follow-up instructions.
Stop retries after a permanent rejection
A 550 response is a permanent failure for that delivery attempt. Stop automatic retries and pause the affected sending stream while investigating. S3150 is a policy rejection, so it does not establish that the recipient address is invalid. Resume delivery only after corrective action and a successful controlled test.
Ticket evidence template (replace sample values)
Hello Microsoft support, We are seeing S3150 rejections for legitimate mail from 203.0.113.10. The mail is transactional, requested by recipients, and authenticated. Sample 1: 2026-08-10 03:14 UTC, user@outlook.com, 550 5.7.1 S3150 Sample 2: 2026-08-10 03:22 UTC, user@hotmail.com, 550 5.7.1 S3150 Sample 3: 2026-08-10 03:31 UTC, user@live.com, 550 5.7.1 S3150 Attach the complete raw NDRs for these samples, including receiving host and diagnostic identifiers. Authentication: [results from a separate accepted test] DNS checks: [record completed PTR and HELO/EHLO checks] Corrective action: [describe completed changes] Please review the continuing S3150 rejection for the affected IP.
What to fix before requesting delisting
Before asking for removal, make the sending identity consistent. Microsoft should see a stable IP, clear reverse DNS, authenticated mail, expected volume, and recipients who asked for the message. That does not guarantee delisting, but it removes obvious reasons for denial.
Fix first
- Authentication: SPF and DKIM pass. For DMARC to pass, at least one authenticated domain must match the visible From domain under the published DMARC matching rules.
- Identity: Use valid PTR records, a hostname that resolves back to the sending IP, and a valid fully qualified hostname in HELO/EHLO.
- Audience: Suppress invalid addresses, unsubscribes, and complaints. Review old or inactive lists before sending; a role address alone is not proof of invalidity.
- Cadence: Keep volume stable and avoid resending large backlogs into Microsoft inboxes.
Limits of common fixes
- DMARC alone: Passing DMARC helps, but S3150 is usually an IP or network block decision.
- Content swaps: Changing templates rarely clears S3150 unless the content caused complaints.
- New tickets: Follow the existing case's reply instructions and include fresh evidence before opening duplicate requests.
- Warmup myths: Warmup helps reputation, but it does not force Microsoft to remove a block.
Illustrative authentication recordsDNS
example.com TXT "v=spf1 include:send.example.net -all" selector._domainkey.example.com TXT "v=DKIM1; k=rsa; p=..." _dmarc.example.com TXT "v=DMARC1; p=none; rua=mailto:d@example.com"
These records are examples, not publishable configuration. Use your provider's real SPF include and DKIM public key. The DMARC value p=none requests monitoring; keep an existing quarantine or reject policy rather than weakening it to troubleshoot S3150. Outlook.com's separate high-volume authentication failure uses 5.7.515, so do not treat that code as an IP delisting issue.
What Microsoft 365 message trace can show
Microsoft 365 email investigation view for reviewing delivery failures
If you control a Microsoft 365 recipient tenant, use message trace in the Exchange admin center to investigate messages the service processed. The message trace link in Microsoft Defender opens that Exchange view. Connection-level IP blocks are not recorded in message trace, and an S3150 rejection normally leaves no quarantine item to release.
For sender-side troubleshooting, keep the SMTP transcript and connecting IP. For tenant-side troubleshooting, compare the bounce timestamp with trace results, allowing for reporting delays and checking the envelope sender and search filters. An absent trace alone does not prove where rejection happened. Microsoft 365 tenant tracing does not investigate personal Outlook.com or Hotmail mailboxes.
How to resume sending after delisting
A delisting confirmation is a reason to test, not proof that every sending IP has recovered. Verify SMTP acceptance for each affected IP before restoring normal traffic.
- Send a fresh test to a mailbox you control through each previously blocked IP. Record the SMTP response and check mailbox arrival; acceptance alone does not guarantee inbox placement.
- Resume recent, expected mail at a reduced rate. Increase volume gradually and avoid releasing the entire incident backlog at once, especially expired password resets or outdated notifications.
- For temporary 4xx deferrals, retry with backoff and reduce throughput or connection concurrency when the response indicates rate limiting. Stop automatic retries if 550 S3150 returns, and add the fresh failure to the support case.
- Review recipients suppressed specifically because of S3150. Restore eligible addresses after the block is resolved, while retaining suppressions for invalid addresses, unsubscribes, and complaints.
The cited discussions do not establish a universal S3150 hourly limit or messages-per-connection cap. Set sending rates using the affected stream's SMTP responses rather than copying another sender's numerical limits.
Where Suped fits
Suped is our DMARC and email authentication platform. In an S3150 incident, use its reporting to compare known sending sources with their authentication history while the Microsoft case is open. DMARC reports do not remove an S3150 block.
Blocklist monitoring page showing domain and IP checks across blocklists with importance and status
In Suped, review reported SPF, DKIM, and DMARC results by sending source alongside your sending logs. Suped's blocklist monitoring tracks public blacklist changes that warrant investigation alongside the incident. It does not expose Microsoft's internal blocklist or confirm S3150 removal.
Mail rejected before message content is accepted can be absent from DMARC aggregate reports. Use the sending server's SMTP logs to confirm the affected IPs and recovery; a clean authentication report alone does not establish successful delivery.
When to escalate a suspected filtering issue
A sudden S3150 block across previously stable sending IPs warrants escalation with timestamped evidence. Clean public blacklist checks and successful delivery elsewhere do not prove that Microsoft's filtering is at fault. Document the pattern and request review of the affected IPs.
Signs to escalate
- Sudden start: S3150 begins across Microsoft domains without a matching change in volume or content.
- Clean checks: Public blacklist checks are clean and authentication passes on the affected stream.
- Moving target: One IP clears, then a different IP in the same operation starts seeing S3150.
- Support clue: A support reply references technical difficulties or asks for more current examples.
When blocks keep returning after delisting, compare the pattern against deeper Microsoft reputation issues. The related pages on a Microsoft bounce message and Microsoft IP delisting cover those cases in more detail.
Views from the trenches
Best practices
Keep raw bounces with IPs and UTC times so Microsoft can review the exact S3150 events.
Follow the case's reply instructions and send fresh samples when support misses the block.
Check public blacklist status, but treat clean results as only one signal in the review.
Stop automatic retries after S3150, then collect fresh evidence through controlled tests.
Common pitfalls
Assuming DMARC failure caused every S3150 bounce leads teams away from IP evidence.
Opening repeated tickets without new bounce samples slows review and weakens the case.
Clearing one affected IP does not prove the entire sending range has fully recovered.
Large retry queues can make Microsoft see more unwanted traffic during the incident.
Expert tips
Track first-seen and last-seen times per sending IP so moving blocks are easy to explain.
Send a short ticket with samples, authentication status, and the mail stream purpose.
Compare Microsoft failures against other providers before changing templates or DNS.
Use alerting on reputation and authentication so S3150 is caught before volume grows.
Marketer view
Marketer from Email Geeks says a burst of S3150 bounces cleared quickly after support was contacted, which points to ticket escalation being necessary in some cases.
2025-08-20 - Email Geeks
Marketer view
Marketer from Email Geeks says S3150 can appear on one group of IPs, disappear, and then show up on other IPs, so teams should track each affected sender separately.
2025-08-20 - Email Geeks
Frequently asked questions
S3150 identifies an IP or network block at Microsoft, not a DMARC failure diagnosis. Check SPF, DKIM, and DMARC separately; passing authentication does not automatically clear the block.
Yes. Microsoft has its own internal reputation data. A clean public blacklist result helps your case, but it does not guarantee Microsoft acceptance.
Do not rotate IPs just to bypass the block. Fix the sending stream, collect evidence, and request review. Moving the same traffic to a new IP can spread the reputation problem.
Send several recent samples with UTC timestamps, recipient domains, sender domain, affected IP, and the full SMTP response. Fresh examples are more useful than old screenshots.
Microsoft does not publish a fixed S3150 resolution time. Check fresh delivery results for each affected IP and follow the case's instructions if rejection continues. Timelines for Microsoft's separate Microsoft 365 delisting process are not an S3150 guarantee.
