New AI Answers: get named when buyers ask ChatGPT, Perplexity and Gemini who to hire. Watch 77-second video

November 4, 2026 · LegalBriefsUSA

Why Is Microsoft Blocking My Cold Emails? It Checks IPs and Each Buyer’s Own Policy, and Inbox Placement Trails Gmail by 11.6 Points

There are 4 places where Microsoft can stop a cold email: the reputation of your sending IP, an authentication rule on Outlook.com, the spam policy the recipient’s company has set, or your own Microsoft 365 account. In most cases the bounce code tells you which of these fired. If nothing bounces, the mail was accepted, and the recipient’s policy then moved it to Junk or quarantine.

Validity’s 2024 seed data put inbox placement at 75.6% for Microsoft and 87.2% for Gmail, and the report names Microsoft as the hardest of the major providers. This guide belongs to our cold email deliverability hub. It ties each bounce to its cause, points to the header that names the filter, and keeps what Microsoft actually documents apart from what vendors keep repeating. Each outside figure is linked and dated at the bottom.

Key takeaways

Why does Microsoft block my cold emails when Gmail delivers them?

The two providers look at different signals and publish different rules. Google sets out a spam-rate line and reports reputation per domain. Microsoft’s Outlook.com policy page gives no such line, its sender dashboard is organized by IP, and every business customer adjusts its own filter.

Factor Gmail Microsoft
Inbox placement, 2024 seed data 87.2% 75.6%
Published complaint line Below 0.3%, target 0.10% None on the Outlook.com policy page
Sender dashboard Postmaster Tools, per verified domain, with domain and IP reputation SNDS, per IP, data may be absent under 100 messages a day
High-volume rule Close to 5,000 a day to personal Gmail More than 5,000 a day to Outlook.com accounts
Filter on company mail Published rules cover personal Gmail only Each customer’s anti-spam policy, bulk threshold 7, 6 or 5

Based on Validity, Google and Microsoft documentation, checked 3 November 2026. Validity’s numbers come from customer test campaigns rather than cold outreach, so treat the gap as a direction and not a forecast.

Outlook.com and Microsoft 365 are also separate doors. Outlook.com handles consumer addresses like hotmail.com and live.com. Microsoft 365 handles company domains, and that is where most B2B lists point. Microsoft says its business filtering learns from Outlook.com user feedback, so the two are linked, but they have different rules and different fixes.

What does each Microsoft bounce code mean?

Each code tells you whose problem you are looking at. Some point to your IP, one to your authentication, one to the recipient’s policy, and the rest to your own Microsoft 365 tenant.

Code Meaning Where it fired First move
550 5.7.1 (S3150) Part of the sender’s network is on Microsoft’s block list Outlook.com, IP reputation Request delisting; a Microsoft Q&A moderator says reputation blocks are not always lifted immediately
550 5.7.606 to 5.7.649 Sending IP is blocked Microsoft 365, IP Use sender.office.com; allow 24 hours or longer
550 5.7.511 Banned sender, portal cannot clear it Microsoft 365, IP Forward the bounce to delist@microsoft.com; a reply comes within 48 hours
550 5.7.515 Domain lacks the required authentication Outlook.com, 5,000 a day rule Fix SPF, DKIM and DMARC alignment
550 5.7.513 Rejected by the recipient company’s own policy (per Suped) Recipient tenant Stop mailing that domain
550 5.1.8 Bad outbound sender, your mailbox is restricted Your tenant Check for compromise, then an admin unblocks it; 1 to 24 hours
550 5.7.705 Tenant exceeded threshold on spam or bulk mail Your tenant Fix the cause, then contact Microsoft Support
550 5.7.708 Traffic not accepted from this IP; more likely on new or trial tenants Your tenant Ask Microsoft Support for an exception if on a trial
550 5.7.233 Tenant hit its daily external recipient limit Your tenant All external sending stops until the count falls

Taken from Microsoft Learn, apart from 5.7.513, which comes from Suped, a deliverability vendor. Checked 3 November 2026.

S3150 needs a word of caution. In a Microsoft Q&A thread from August and September 2026, one sender’s delisting request was answered with “Not qualified for mitigation,” after which sender.office.com showed the IP as not blocked even though the bounce kept coming. As of the last reply on 7 September, no resolution had been posted. Expect this to take days, not hours.

Why does my email land in Junk at Microsoft with no bounce?

The recipient company’s anti-spam policy took the message in and then dealt with it. On default settings, spam and bulk mail go to Junk. The Standard preset quarantines high confidence spam, and Strict quarantines all spam. A quarantined message never reaches the mailbox unless someone releases it, and nothing tells you it happened.

Bulk mail gets a score from 0 to 9 called the bulk complaint level, or BCL. Microsoft does not say how it calculates the score, only that it draws on internal and external sources.

BCL Microsoft’s description Default policy (threshold 7) Strict preset (threshold 5)
0 Not from a bulk sender Passes Passes
1 to 3 Bulk sender, few complaints Passes Passes
4 to 7 Bulk sender, mixed complaints Junk at 7 Quarantine at 5 and above
8 to 9 Bulk sender, high complaints Junk Quarantine

Based on Microsoft Learn, checked 3 November 2026. “Passes” means the bulk rule took no action; other filters can still act.

Two other behaviors tend to surprise outbound teams. Zero-hour auto purge is on by default and can act on a message after delivery, so something that reached the inbox may not stay there. The first contact safety tip, which reads “You don’t often get email from” followed by your address, is recommended on in both Standard and Strict. It does not block anything, but it puts a warning on top of your opening line.

How do I find out which filter stopped my email?

Send the campaign to a Microsoft 365 mailbox you control and look at the headers. The CAT field names the policy that was applied. This shows your own tenant’s verdict rather than the buyer’s, but it is enough to tell a reputation problem from an authentication problem.

Header field What it tells you
CAT in X-Forefront-Antispam-Report Policy applied, such as BULK, SPM, HSPM or SPOOF
SFV in the same header SFV:SPM means marked as spam, SFV:NSPM means nonspam
CIP in the same header The connecting IP, which is the address to check or delist
BCL in X-Microsoft-Antispam Bulk complaint level, 0 to 9
compauth, spf, dmarc in Authentication-results Whether Microsoft treated the From domain as authenticated

Disregard advice that relies on SCL scores. Microsoft says SCL does not decide, in cloud organizations, whether a message is treated as spam or what happens to it, and it tells admins to look at CAT and DIR instead.

Do Microsoft 365 sending inboxes cause the blocks?

They can, since your own tenant carries limits that have nothing to do with the recipient’s filters. At cold email pace, volume is seldom what triggers them. The real risk is outbound spam verdicts, and Microsoft says it blocks accounts that go past “an undisclosed limit.”

Microsoft also says that sending bulk email through Microsoft 365 “isn’t a supported use of the service” and is allowed only on a best-effort basis. The risk grows when cold inboxes sit in the same tenant as your staff, because a block at tenant level stops everyone. It is another reason to keep cold email off your main domain and its tenant.

Can I see my reputation at Microsoft?

Only in part. SNDS reports filter results at the IP level: green below 10% spam, yellow from 10% to 90%, and red above 90%. Microsoft cautions that IPs sending fewer than 100 messages on a given day may have no data, and Litmus points out that you have to own the IP to register it.

For most cold programs, that means flying blind. A domain running 3 inboxes at 25 a day sends 75 messages, which is below the 100 line, and anyone sending on shared Google or Microsoft infrastructure does not own the IP. If you are able to enroll, JMRP, Microsoft’s free complaint feedback program, can send reports in as little as 72 hours.

A clean dashboard does not prove much either. In March 2026, The Register reported that a company’s static IPs were being rejected while SNDS showed no issues and JMRP showed no spam reports. Microsoft later described it as “a temporary rate-limit issue.”

Which Microsoft numbers online should I ignore?

Ignore any complaint threshold, open-rate floor or domain-age number presented as Microsoft policy. We did not find any of them in the Microsoft pages we checked, and Microsoft says it keeps exact limits private “so spammers can’t game the system.”

FirstSales, which sells cold email software, says complaint rates over 0.10% “trigger junk routing,” tells senders to cut volume if open rates stay under 8%, wants a domain to be 14 days old before a campaign, and says the default BCL threshold in many configurations is “6 or lower.” It gives no Microsoft source for any of this. Microsoft’s own default BCL threshold is 7, and 6 is the Standard preset. Read the other figures as practitioner rules of thumb, not as rules.

The 5,000 rule is often overstated as well. It applies to Outlook.com accounts, and a domain sending 75 a day is at 1.5% of it. Our breakdown of Google bulk sender requirements puts the provider rules side by side, line by line.

How do I fix a Microsoft block?

Start with the code and work outward, and hold the affected sends until you are done. Microsoft’s policy page tells senders not to retry after a permanent failure.

The split matters because no single provider dominates. One analysis of the Tranco top million found Google Workspace behind 21.8% of detectable MX records and Microsoft 365 behind 16.8%, although those are general domains and not B2B lists.

How LeadButton handles this

We send from separate registered domains and never from subdomains of a client’s brand, so if Microsoft blocks a sending domain, the mail your company depends on is untouched. Each domain has SPF, DKIM and aligned DMARC in place before it sends a single message, new inboxes ramp up over three to four weeks, personal Outlook.com addresses are removed from B2B lists, and a sequence stops as soon as anyone replies. Launch costs $1,500 a month for managed email outreach, Growth costs $3,500 a month for email plus LinkedIn, and Scale is quoted for bigger programs. You can see the tiers on the LeadButton pricing page.

Frequently asked questions

What does S3150 mean?

It is a 550 5.7.1 rejection stating that part of the sending network is on Microsoft’s block list. A Microsoft Q&A moderator calls it a reputation issue that does not always get lifted right away.

Does Microsoft publish a spam complaint limit?

Not on its Outlook.com policy page. The SNDS FAQ only notes that more than 30% of IPs keep complaint rates below 0.3%, which is a point of reference and not a rule.

How long does Microsoft delisting take?

Through the portal it can take 24 hours or more. For 5.7.511, Microsoft says it will reply with next steps within 48 hours.

Why does SCL not explain my result?

Microsoft says that in cloud mailboxes SCL decides neither the verdict nor the action. Look at CAT and BCL instead.

Does a 5,000 a day rule apply to my cold domain?

Almost never. It applies to domains sending over 5,000 a day to Outlook.com accounts, which is far more than a typical cold domain sends.

Sources

Leave a Reply

Back home