The problem
"Please contact your Internet service provider" — for a block Microsoft won't explain
Here is a real, lightly-anonymised bounce message. The sending server had valid SPF, DKIM and DMARC, a clean sending history, and no listing on any public DNS blocklist. Microsoft's own postmaster tools (SNDS) showed nothing wrong either. The message still bounced:
S3150 is issued by Microsoft's internal Protocol Filter Agent (PFA) — an automated system whose blocklist is private, cannot be queried directly, and comes with no explanation of what triggered it. The advice to "contact your Internet service provider" is frequently meaningless: the provider's network is often clean, and the block was applied to a shared range, a single IP, or a whole prefix for reasons Microsoft support staff themselves are often unable or unwilling to disclose.
No explanation
Support staff routinely tell administrators "we do not have the liberty to discuss the nature of the block."Hours to days
Typical turnaround through the delist portal, when it works — sometimes with no confirmation email at all.It comes back
Administrators report the same server being re-blocked every few months, with the delisting process starting over each time.What to do about it
Two starting points
I use a hotmail.com, outlook.com or live.com address
If your mail address depends on Microsoft, other people's messages to you are the ones at risk of silently vanishing — and you'll likely never know it happened. See why, which providers are worth switching to, and how to move without losing a single email.
Read the user guide →I administer a mail server or a website
Mail arriving from a Microsoft-hosted address deserves a clear heads-up back to the actual sender, not silent acceptance — and web forms that promise a confirmation email to an @outlook.com address are setting up a support ticket. Here's how to configure Postfix, Exim, Sendmail and web forms accordingly.
Read the administrator guide →Further detail
Why properly-run mail servers still get blocked
None of this requires misconfiguration on the sender's side. The recurring pattern reported by administrators and journalists alike:
Opaque criteria
Blocks are applied by an internal reputation system whose rules, thresholds and evidence aren't published or queryable by the sender.
Shared-range collateral
A block on part of a netblock or a single misbehaving neighbour on shared infrastructure can take an entire, well-behaved range down with it.
Delisting is manual
The public delist portal frequently errors out, gives conflicting timelines (30 minutes to 72 hours), and sends no confirmation.
No lasting fix
Because the underlying signal is never disclosed, the same server can be re-flagged again a few weeks or months later.
References & further reading
This is not an isolated complaint. A sample of independent reporting and public discussion from administrators dealing with the same thing:
Users fume at Outlook.com email 'carnage' — The Register
Reports weeks of legitimate mail — invoices, delivery notices, authentication codes — being rejected or silently dropped for Outlook.com, Hotmail, Live and MSN recipients, with Microsoft eventually citing a "temporary rate-limit issue" after days of silence.
theregister.com · March 2026Emails to Outlook.com Rejected By Faulty Or Overzealous Blocking Rules — Slashdot
Community discussion of the same incident, aggregating first-hand reports from affected senders and administrators.
slashdot.org · March 2026NANOG mailing list: erroneous Outlook.com block of a full /25
A network operator reports an entire announced prefix erroneously added to Microsoft's blocklist, and difficulty reaching "a clueful person at MS" to resolve it. Public NANOG archive, dating back to 2015 — this is a long-running pattern, not a one-off.
diswww.mit.edu/charon/nanog · archived thread"Microsoft/Outlook blocking your mail server?" — DirectAdmin Forums
Hosting administrators compare notes on servers being blocked "every few months" with no explanation beyond "we do not have the liberty to discuss the nature of the block," and on Microsoft mangling DKIM-signed headers in a way no other provider does.
forum.directadmin.com"My mail server got blocked by Outlook.com" — ParanoidPenguin.net
A self-hosted mail administrator's first-hand account of getting caught by Outlook.com's filtering and working through the delisting process.
blog.paranoidpenguin.netCornell University IT: mail to Hotmail.com, Outlook.com and other Microsoft domains blocked
A university IT department's public service-alert record of an outbound delivery failure to Microsoft-hosted addresses affecting its mailing lists.
itservicealerts.hosting.cornell.eduMicrosoft's own support forum: dozens of open "S3150 block list" threads
Search Microsoft Q&A for "S3150" or "block list" and you'll find a steady stream of administrators reporting the exact same bounce, the exact same lack of explanation, and the exact same manual delist workaround — on Microsoft's own platform.
learn.microsoft.com/answers