What should I do if Microsoft blocks my IP address during IP warming?
Published 23 May 2025
Updated 10 Oct 2026
13 min read
Summarize with
Updated on 10 Oct 2026: We clarified Microsoft IP block recovery, authentication failures, and the checks needed before restarting affected sends.
If Microsoft blocks a warming IP, pause new sends on the affected Microsoft recipient routes immediately, keep other sends running only if they remain healthy, pull the raw SMTP rejection, and involve the IP owner or ESP in a mitigation request. Do not keep sending to Outlook.com, Hotmail, MSN, Live, or affected Microsoft 365-hosted addresses just to stay on the warm-up schedule. Rejected messages cannot reach recipients or generate engagement.
The key detail is the exact bounce. A generic ESP label like "blocked due to spam or sender reputation" is not enough. A hard S3150 or S3140 block calls for a different response than a temporary S775 rate limit.
- Pause affected routes until the raw rejection is understood. Check consumer Microsoft services and Microsoft 365 separately.
- Capture raw SMTP replies, timestamps, sending IPs, recipient domains, campaign IDs, volumes, and complaint signals.
- Involve the ESP or network that controls the IP so the mitigation request includes routing and network evidence.
- After the restriction clears, restart the affected Microsoft volume below the failed level and increase only with clean signals.
Pause sends on the affected Microsoft routes
Continue warming only at healthy recipient providers, pause new sends on the affected Microsoft routes, and open the correct Microsoft support path with the ESP or IP owner. Stop the entire warm-up if the same IP also shows serious blocks across other recipient providers or private domains. A policy block does not prove that a recipient address is invalid, so keep route pauses separate from permanent invalid-address suppressions.
Do not keep sending to affected Microsoft routes through a hard block. If Microsoft rejects mail at SMTP time, those deliveries cannot generate positive recipient engagement. During warming, Microsoft evaluates the IP's behavior, list quality, recipient engagement, complaint rate, and historical patterns closely.
Treat Microsoft as its own warm-up lane
A 99.6% overall delivery rate can hide a Microsoft-specific failure if Microsoft is only 8% of the list. Segment results by recipient domain and mail route before deciding that the whole plan is healthy.
Continuing all sends
- More sends through the same hard block produce more rejected deliveries.
- Rejected messages give recipients no chance to engage.
- Repeated manual resends add workload without fixing the reason for rejection.
Pausing affected Microsoft routes
- New sends stop on the affected route while the case is investigated.
- You can give Microsoft exact bounce text, volumes, and a corrected ramp plan.
- You restart the affected route at a lower volume instead of resuming at the failed peak.
Pull the raw Microsoft rejection
Before changing the warm-up schedule, get the raw rejection string. ESP bounce categories often compress several different Microsoft events into one label. The raw SMTP response tells you whether the problem is a hard IP block, a temporary rate limit, an authentication or content-related rejection, or a recipient-level problem.
In an ESP data warehouse, query bounce data instead of relying on the dashboard category. Capture the sending IP, account or business unit, job ID, recipient domain, event date, bounce category, SMTP code, and full SMTP bounce reason. The Salesforce Marketing Cloud _Bounce query below does not return the sending IP and is not a complete log of temporary SMTP deferrals. Obtain those details from the ESP or MTA logs, and include regional Microsoft domains such as hotmail.de and outlook.de plus custom domains identified by their delivery route.
Salesforce Marketing Cloud _Bounce query for Outlook and Hotmail SMTP rejection data.
Example bounce querySQL
SELECT SubscriberKey, JobID, Domain, EventDate, BounceCategory, SMTPCode, SMTPBounceReason FROM _Bounce WHERE EventDate >= DATEADD(day, -3, GETDATE()) AND ( Domain IN ('outlook.com', 'hotmail.com', 'msn.com', 'live.com', 'outlook.de', 'hotmail.de', 'live.de') OR SMTPBounceReason LIKE '%S3150%' OR SMTPBounceReason LIKE '%S3140%' OR SMTPBounceReason LIKE '%S775%' OR SMTPBounceReason LIKE '%4.7.650%' OR SMTPBounceReason LIKE '%5.7.515%' OR SMTPBounceReason LIKE '%5.7.606%' OR SMTPBounceReason LIKE '%5.7.511%' )
Illustrative Microsoft rejection patterns
550 5.7.1 ... part of their network is on our block list (S3150). 550 5.7.1 ... part of their network is on our block list (S3140). 451 4.7.650 ... temporarily rate limited due to IP reputation (S775). 550 5.7.515 ... sending domain does not meet the required authentication level.
Choose the response by SMTP error code
Once the raw rejection is visible, the next move becomes clear. Treat confirmed Microsoft IP or network blocks as a stop condition for the affected route. For temporary rate limits, reduce new Microsoft volume sharply, increase spacing, and let the sending MTA retry according to its queue policy while watching whether deferrals recover. Do not manually resend permanent 5xx failures before correcting the cause, and do not add new campaigns on top of a growing deferred queue.
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|---|---|---|
S3150 | IP block | Pause and escalate |
S3140 | IP or network block | Pause and investigate with the IP owner |
S775 | Temporary rate limit | Throttle and retry |
550 5.7.515 | Sending-domain authentication failure | Fix message authentication before resuming |
Generic spam | Needs detail | Pull raw reply |
Use the raw Microsoft SMTP response, not the ESP category, to choose the next action.
A 550 5.7.515 rejection concerns the sending domain's authentication, so an IP delisting request does not fix it. Microsoft's Outlook.com authentication requirements apply to high-volume domains sending 5,000 or more messages a day to its consumer services using the same visible From domain. Both SPF and DKIM must pass. A valid DMARC policy of at least p=none is required, and DMARC must pass through SPF or DKIM with the authenticated domain meeting DMARC's domain matching rules for the visible From domain. Check a real message's Authentication-Results header, because published DNS records alone do not prove that messages pass.
For deeper troubleshooting, separate hard S3150 bounces from temporary 451 4.7.650 errors. They share an IP reputation theme, but the recovery path is not identical.
Microsoft response stop conditions
Use the full SMTP response and repetition pattern rather than an overall bounce percentage. Acceptance does not guarantee inbox placement.
Clean acceptance
Stable 2xx
SMTP acceptance without a repeating block pattern
Throttle
Repeated 4xx
Repeated temporary reputation deferrals
Pause
5xx IP or network block
Confirmed IP or network policy rejection
Choose the right Microsoft delisting path
Microsoft uses different support paths for consumer Outlook.com infrastructure and Microsoft 365 tenant protection. Follow the instruction in the full NDR instead of submitting the same IP to every form. Using the wrong path can return a clean result even while another Microsoft system is rejecting the IP.
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|---|---|---|
S3150 or S3140 to Outlook.com, Hotmail, Live, or MSN | IP, full NDR, domains, and corrected ramp | |
5.7.606-649 banned sending IP | Email address that received the NDR and one blocked IP | |
5.7.511 | Forward the NDR to delist@microsoft.com | Full NDR code and blocked IP |
Match the recipient system and exact NDR to the supported Microsoft route.
For 5.7.606-649, enter the email address that received the NDR and the blocked IP, confirm the verification email, then select 'Delist IP'. Microsoft says removal takes up to 24 hours or longer, so verify acceptance before increasing volume. For 5.7.511, the portal cannot remove the block: forward the full NDR and IP address to delist@microsoft.com. These are the steps in Microsoft's Microsoft 365 delisting instructions.
Do not treat a clean portal result as clearance
A portal can report that an IP is not on one blocked-sender list while Outlook.com still rate limits or rejects it on reputation grounds. Keep the automated response, reply with the complete NDR when the support workflow allows it, and keep new sends on the affected route paused until acceptance is verified.
Verify the outbound IP and network
Use the public sending IP in the raw SMTP reply, not your website IP or an IP shown only in account settings. Confirm with the ESP whether Microsoft-bound mail actually uses the warming dedicated IP, a shared IP pool, or another outbound relay. Separate results by sending IP before attributing every failure to the new IP.
- If the NDR says part of the provider's network is blocked, ask the IP owner to investigate the affected range and other senders in the same pool.
- On self-managed servers, check for compromised accounts or an open relay. Use a stable sending IP with valid reverse DNS, not a dynamic residential IP.
- Check that the PTR hostname resolves back to the sending IP and that EHLO uses a valid server identity. The PTR hostname does not have to equal the visible From domain.
- An automated warm-up marked complete is not proof that Microsoft accepts the IP. Continue measuring Microsoft-specific acceptance before increasing volume.
Microsoft's sender troubleshooting guidance explains its reverse-DNS and dynamic-IP checks. If essential transactional mail needs another route during the pause, use an existing healthy, authenticated route with the provider's help and within its normal capacity. Switching to another unwarmed IP starts a new reputation-building task.
Check reputation signals around the block
A Microsoft block during warm-up often combines volume timing, list quality, IP history, and engagement. Check the blocklist (blacklist) state, message authentication, Microsoft-specific complaint behavior, and whether the warm-up jump was too sharp for that mailbox family.
The IP can be clean on public blocklists and still face a Microsoft-specific block. Separate public blacklist data from Microsoft acceptance data instead of treating a clean public result as proof that the IP has healthy Microsoft reputation.
Blocklist checker
Check your domain or IP against 144 blocklists.
For IPs you own or have permission to monitor, review Microsoft's Smart Network Data Services (SNDS) and Junk Mail Reporting Program (JMRP) feedback. Look for complaint activity, abnormal volume, and IP status changes. The current SNDS portal has replaced the legacy site, so verify network access and automated data feeds. Microsoft removed trap-hit counts from the Data Report on July 22, 2026; their absence is not evidence of a trap-free list. Missing SNDS data does not prove that an IP is healthy, so compare it with raw SMTP results.
Run a broader domain health check and send a real message through an email tester before asking Microsoft to mitigate. Verify SPF, DKIM, and DMARC on actual messages and check rDNS and EHLO identity before resuming affected Microsoft volume.
Blocklist monitoring page showing domain and IP checks across blocklists with importance and status
Suped's product combines DMARC reporting with blocklist monitoring and alerts. Use it to identify failing authentication sources and public blacklist listings, then document corrective work alongside the ESP's Microsoft bounce evidence. Microsoft-specific acceptance still needs to be checked in the delivery logs.
Work through the IP owner
If the IP is controlled by an ESP or cloud network, involve the IP owner. The provider can confirm the actual outbound route, investigate shared-pool or network issues, and coordinate mitigation. If you administer your own sending infrastructure, the person most familiar with it can submit the appropriate Microsoft support request directly.
Include a clear Microsoft-only ramp plan in the mitigation request; there is no general pre-approval that guarantees delivery. Microsoft's warm-up process guidance confirms that recipient engagement and sender reputation need to build gradually.
- Identity: sending domain, envelope domain, DKIM domain, return-path, and the affected IP address.
- Evidence: raw SMTP replies, affected Microsoft domains, first failure time, and send volume by day.
- Quality: opt-in source, suppression process, complaint review, and bounce cleanup since the block.
- Ramp: a smaller Microsoft-only schedule with engaged recipients first and no sudden jumps.
Where Suped helps
Suped's DMARC reporting helps identify sending sources and authentication failures across your domains. During a Microsoft block, use those results to verify corrections and collect authentication evidence for support. Combine them with the ESP's raw SMTP replies and recipient-route volumes.
Rewarm Microsoft separately
After mitigation, verify acceptance with a small test before increasing volume. Do not resume at the failed peak. Treat Microsoft as a separate destination pool and restart below the last successful Microsoft volume. For example, if the block happened at a total send of 25,000 and Microsoft-routed recipients were about 8% of the list, the Microsoft volume was around 2,000. Restart well under that number with recently engaged Microsoft recipients first.
Illustrative Microsoft restart, not a fixed schedule
Example starting point: 250 recently engaged Microsoft recipients Hold at 250 until acceptance and queue recovery are stable Example later steps: 500, 750, 1,000, then 1,500 recipients Spend multiple days at each step when the signals need more time Reduce new sends if deferrals persist; pause on a renewed IP block
Example Microsoft rewarm volume
Illustrative daily volumes only, not a Microsoft-approved schedule. Hold or reduce a day's volume when deferrals or complaints increase.
Microsoft recipients
The exact numbers depend on list size and previous acceptance, but the pattern matters more than the sample values: resume below the failure point, prioritize recent clicks, purchases, and account activity, hold volume steady or reduce new sends when Microsoft defers mail, and increase only after clean acceptance. Treat opens as a supporting signal because privacy controls can distort them. Microsoft does not publish one universal restart volume or completion deadline. SMTP acceptance also does not prove inbox placement, so review complaints and recipient engagement after messages are accepted.
Prevent another Microsoft IP block
The best fix goes beyond delisting. It makes the next Microsoft send predictable. Use lower Microsoft ramp steps, better recipient selection, clean authentication, and a feedback loop that surfaces block patterns before a whole send day is wasted.
- Track consumer Microsoft services and Microsoft 365 custom domains separately, then review bounces by sending IP and delivery route.
- Warm Microsoft with recent clickers, buyers, account users, and carefully interpreted openers before older contacts.
- Avoid sudden Microsoft volume increases even when the total warm-up curve looks reasonable.
- Keep SPF, DKIM, DMARC, rDNS, EHLO, and tracking domains consistent through migration.
- Monitor per-route bounces so Microsoft blocks surface before the next scheduled increase.
Microsoft IP warming recovery flow: pause affected sends, check SMTP errors, request mitigation, and rewarm.
Views from the trenches
Best practices
Pull raw SMTP replies before changing the ramp, because ESP categories hide key details.
Pause new sends on affected Microsoft routes, then keep healthy destination pools under review.
Involve the IP owner in mitigation so the support case includes network and routing evidence.
Restart with engaged Microsoft recipients and grow only after stable acceptance returns.
Common pitfalls
Continuing sends through a Microsoft hard block produces rejections without recipient engagement.
Treating overall delivery as healthy hides Microsoft failures when that segment is smaller.
Submitting mitigation without a Microsoft-only ramp plan leaves the next step unclear.
Relying only on public blacklist status misses Microsoft's private IP reputation decisions.
Expert tips
Automate bounce extraction early so raw Microsoft responses are available during incidents.
Track Microsoft routes as a separate cohort rather than mixing them into total delivery.
Document opt-in source and suppression logic before asking Microsoft for mitigation.
Hold or reduce new sends after persistent deferrals rather than increasing on a fixed calendar.
Marketer view
Marketer from Email Geeks says Microsoft can be sensitive during careful warm-ups, so the exact bounce text should guide every recovery decision.
2024-12-16 - Email Geeks
Marketer view
Marketer from Email Geeks says ESP bounce categories are not reliable enough for planning, and the raw rejection must be reviewed first.
2024-12-16 - Email Geeks
Frequently asked questions
Stop new sends on the affected Microsoft routes first. Keep other destinations only if their bounce, complaint, and engagement signals remain healthy. Stop the whole plan if the same IP shows broad provider-level rejection. Diagnose consumer Outlook.com and Microsoft 365 separately.
No. Messages rejected by a Microsoft hard block cannot reach recipients or generate engagement. Keep new sends on affected routes paused until the cause is corrected and a small test confirms acceptance. For temporary 4xx deferrals, reduce new volume and let the MTA retry within its queue policy.
Involve the ESP or network owner when it controls the sending IP. For self-managed infrastructure, the administrator who understands the sending system can contact Microsoft directly. Supply raw rejection evidence, the permission source, complaint review, and the corrected Microsoft-only ramp plan.
Use the path named in the full NDR. S3150 or S3140 affecting Outlook.com, Hotmail, Live, or MSN belongs in Outlook.com sender support. A Microsoft 365 NDR stating 5.7.606-649 and "banned sending IP" points to the Microsoft 365 delist portal. For 5.7.511, forward the full NDR and IP address to delist@microsoft.com rather than using the portal.
No. Microsoft uses its own reputation signals in addition to public blocklist and blacklist data. A clean public lookup is useful, but it does not override Microsoft-specific rejection data.
Mitigation can reopen delivery, but it is not a clean reputation reset. Resume below the failed volume, use engaged recipients first, and increase only when Microsoft acceptance stays stable.
