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Provider deliverability · All mailbox providers

Why are my emails being blocked?

Samuel Chenard

By Samuel Chenard · CEO & Co-Founder, Palisade · Reviewed September 19, 2026

Emails get blocked for six main reasons: your domain fails SPF, DKIM, or DMARC, your spam complaint rate crossed the provider's threshold, your IP or domain sits on a blocklist, your sending infrastructure is broken, you broke a bulk-sender rule, or the domain has no sending history. Read the bounce to identify the provider, and fix authentication first.

The 30-second check

Start with the symptom. Bounced mail comes back with an NDR naming the blocking server; silently missing mail was accepted and junked. Either way the diagnosis begins with your own domain, because every major provider now gates on the same three records. The free email security score below grades SPF, DKIM, DMARC, and the rest of your domain's mail security in one pass.

Check your domain now

Enter your sending domain and the check runs instantly on the next page. Free, no signup.

Why mailbox providers block your email

Likely causeWhat's happening
Your domain fails SPF, DKIM, or DMARCSince 2024 authentication is the entry ticket everywhere. Gmail and Yahoo require SPF or DKIM to pass for every sender; Gmail demands SPF plus DKIM plus a DMARC policy once you send 5,000+ messages a day, and Yahoo imposes the same trio on bulk senders while explicitly declining to publish a volume threshold; Microsoft enforced the same trio for domains sending 5,000+ daily messages to Outlook.com from May 5, 2025; Apple lists SPF, DKIM, and a published DMARC policy among its bulk requirements for iCloud Mail, with rejection if they're not met (each provider's own documentation, checked 2026-07-17).
Your spam complaint rate crossed the provider's lineGmail tells senders to keep the spam rate in Postmaster Tools below 0.3% and recommends staying under 0.10%; Yahoo publishes the same 0.3% ceiling. Microsoft names the junk-complaint rate one of the principal factors driving a sender's reputation down but publishes no number, and Apple publishes none either; both still filter on user feedback. Past the line, even fully authenticated mail stops arriving.
Your sending IP or domain is on a blocklistMicrosoft runs an internal block list; the tell is a marker like S3150 inside a 550 5.7.1 bounce, and the remedy is Microsoft's self-service delist portal. Public DNS blocklists feed corporate gateways and smaller providers. Shared IPs inherit listings from noisy neighbors, so you can be listed without ever sending a bad message yourself.
Broken sending infrastructure: reverse DNS, TLS, dynamic IPsGmail's guidelines require valid forward and reverse DNS (PTR records) and a TLS connection from every sender, and Apple asks bulk senders to publish reverse DNS that identifies their IPs. Microsoft rejects direct-to-MX mail from dynamic or residential IP space outright with its DY-class codes. These basics fail silently until a provider tightens enforcement.
You broke a bulk-sender list ruleGmail and Yahoo require one-click unsubscribe in marketing mail, and Yahoo requires unsubscribes honored within two days. Apple requires explicitly subscribed recipients only (no purchased, rented, or appended lists) plus ARC headers on forwarded mail. Blasting a stale or bought list also drives the complaint rate that the second cause above punishes.
No sending history: new domain, new IP, or a volume spikeFilters rate-limit senders they have never seen. A new domain or IP starting at full volume reads as a spam cannon and collects deferrals (421-class codes) before outright blocks. No provider publishes a warm-up schedule; Apple tells bulk senders to keep sending IPs and domains consistent, which is the same advice from the other direction.
Five-check diagnosis flowchart for blocked email: a bounce with an NDR means reading the SMTP code, no bounce means the mail was accepted and junked, failing SPF, DKIM, or DMARC means fixing authentication first, passing authentication with one provider still blocking means checking blocklists and complaint data, and a new domain or volume spike means expecting deferrals while warming up.

Check the public signals before changing settings

The free check above gives you a fast public baseline for how providers see your mail. It does not replace the provider's private reputation or placement data, but it tells you whether the problem is visible from outside before you edit a sending platform.

Palisade Email Security Score result showing public DMARC, SPF, DKIM, MX, and reputation checks for a non-sensitive test domain.
Source: Palisade, “Email Security Score”, checked 2026-07-29. First-party public tool result for a non-sensitive test domain; it checks public DNS and reputation signals, not private mailbox placement.

How to fix it, step by step

  1. Run the email security score on your sending domain

    Use the free checker above, or run the Email Security Score. It grades SPF, DKIM, DMARC, and the rest of your domain's mail security in one pass and shows which record is missing, misaligned, or failing, which settles the most common cause before you chase anything else.

  2. Read the bounce, or notice there isn't one

    A rejection returns an NDR with an SMTP code and the blocking server's name; that code plus the SMTP error code reference tells you which provider refused the mail and why. No bounce and no reply usually means silent spam-foldering: send tests to mailboxes you control at each major provider and see where they land.

  3. Fix SPF, DKIM, and DMARC for every sending service

    Add each platform that sends as your domain to your SPF record, enable DKIM signing with your own domain rather than the provider's default, and publish a DMARC record. Verify with the free SPF checker, DKIM checker, and DMARC checker; Gmail, Yahoo, and Microsoft all require the passing domain to align with your From address at bulk volume.

  4. Check the blocklists and request removal

    Run your sending IP and domain through the blocklist checker. For a Microsoft block, submit the IP at sender.office.com; for public lists, follow each list's own delisting process. Fix the root cause first, because delisting a still-broken sender is a round trip back onto the list.

  5. Enroll in the providers' own reputation programs

    Google Postmaster Tools shows your spam rate and reputation with Gmail; Microsoft's SNDS and JMRP do the same for Outlook.com; Yahoo's Complaint Feedback Loop sends you a copy of every complaint. Apple offers no feedback loop; its postmaster contact is icloudadmin@apple.com with your domain, sending IPs, and the SMTP errors you received.

  6. Go provider-specific and re-test

    Once you know who is blocking you, follow the per-provider guides linked below for that filter's quirks: Outlook's SmartScreen and delisting, Gmail's sender requirements, Yahoo's feedback loop, iCloud's bulk rules. After DNS changes propagate, re-send and confirm spf=pass, dkim=pass, and dmarc=pass in the received headers.

Related free tools: SPF checker · DKIM checker · DMARC checker · Blocklist checker · Domain reputation

If you send in volume: the published bulk-sender rules

All four major providers publish bulk-sender requirements; only Gmail and Microsoft attach a volume number to them. Gmail (since February 1, 2024) and Yahoo require SPF or DKIM from every sender. At 5,000+ messages a day Gmail demands SPF plus DKIM plus a DMARC policy of at least p=none that passes with From-domain alignment, one-click unsubscribe, and a spam rate below 0.3%; Yahoo requires the same of bulk senders but states in its sender FAQ that it "will not specify a volume threshold". Microsoft matched Gmail's number on May 5, 2025 for domains sending 5,000+ daily messages to Outlook.com: non-compliant mail was junked first, with rejection to follow. Apple publishes bulk requirements for iCloud Mail without numeric thresholds: SPF and DKIM, a published DMARC policy, ARC headers on forwarded mail, and opt-in-only lists, otherwise "the email will be rejected" (each provider's own documentation, checked 2026-07-17).

Check your standing with each provider

Bounce codes you may be seeing

A block from a provider surfaces as a specific SMTP code. Some lists below also carry codes returned by the systems receiving your mail; each line names its issuer. Match yours below; where a code has a full guide, it is linked.

The real root cause: unenforced authentication

Strip away the provider differences and the same root cause sits under almost every block: mail that can't prove it came from your domain. Unauthenticated mail scores badly, bad scores draw junk placements and complaints, and complaints push you onto the lists that turn filtering into refusal. The way out is to make authentication a settled fact: correct SPF and DKIM for every sending service, DMARC alignment on the From domain, and a policy enforced at p=reject so spoofers can't burn your reputation while you rebuild it. Monitoring shows you which senders fail; enforcement is what makes the blocking stop and stay stopped, at every provider at once.

DMARC software that does the work

Palisade's AI agent takes domains all the way to enforcement: hosted SPF, DKIM, DMARC, and MTA-STS records on paid plans, DMARC reports monitored continuously, and every policy step drafted for your approval on the way to p=reject. The full product is open for a 15-day trial, and nothing is charged until it ends.

Get startedBook a demo

15-day full-product trial. Nothing is charged until it ends.

Fixing this across every client domain

One blocked domain is a support ticket; a fleet of client domains hitting the Gmail, Yahoo, and Microsoft enforcement waves is a recurring queue, because every provider scores every domain separately and each one needs its own SPF, DKIM, DMARC, and postmaster enrollments. Palisade runs this per fleet: it hosts and manages the SPF, DKIM, DMARC, and MTA-STS records for every client domain, flags failing senders from DMARC reports before a provider starts refusing them, and carries each domain to p=reject, with your team approving each step. Tickets land in ConnectWise, HaloPSA, or Autotask through native PSA integrations, pricing is per client domain with rates that improve as the portfolio grows, and your own MSP domain is a free NFR domain to prove it on.

Questions readers ask

Frequently asked questions

How do I find out why my emails are being blocked?

Read the bounce message. Every SMTP rejection returns an NDR containing a code (550 5.7.1, 550 5.7.26, and so on) and usually a URL from the blocking provider. The code identifies both the provider and the reason; the SMTP error code reference decodes each one and links the fix.

Why are my emails being blocked without a bounce message?

Mail that vanishes without a bounce is not blocked, it is junked: the provider accepted the message and filed it in spam, which produces no NDR. Confirm by sending tests to mailboxes you control at Gmail, Outlook.com, and Yahoo. Silent junking points to reputation or content scoring rather than a hard policy failure, so check complaint rates first.

Why are my emails suddenly being blocked in 2026?

Mail that suddenly started bouncing in 2026 met enforcement that caught up with it. Gmail and Yahoo turned their sender requirements on in February 2024 and Microsoft followed for Outlook.com on May 5, 2025, so mail that once landed in spam folders now bounces. If nothing changed on your side, an unauthenticated sending service or a stale list finally met a stricter filter.

How do I check if my email domain or IP is blacklisted?

Run your sending domain and its sending IP through the free blocklist checker, which queries the major public DNS blocklists in one pass. Microsoft's internal list isn't publicly queryable: the evidence is a bounce marker like S3150, and the remedy is the delist portal at sender.office.com. Gmail and Yahoo have no delist portal; use their reputation tools instead.

Why are my emails blocked by one provider but not others?

Each provider runs its own filter and scores you separately: Microsoft's SmartScreen, Gmail's spam classifiers, Yahoo's and Apple's reputation systems share nothing. A complaint spike at one mailbox provider, or a listing only Microsoft consults, can block you in one place while everything else flows. Diagnose per provider with each one's own postmaster tools.

Why are my business emails being blocked by customers' mail servers?

Corporate gateways (Microsoft 365, Google Workspace, Proofpoint, Mimecast and the like) apply the same authentication checks plus tenant-level rules an admin controls. If consumer inboxes accept your mail but one company rejects it, ask their IT contact for the bounce text; if every business rejects it, your SPF, DKIM, or DMARC is failing globally.

Can fixing DMARC stop my emails from being blocked?

Usually, yes, because failed or missing authentication is the one requirement Gmail, Microsoft, Yahoo, and Apple all publish and all enforce. Publish SPF and DKIM for every sending service, align them with your From domain, and enforce DMARC at p=reject so spoofed mail stops damaging your reputation. Blocklist entries and complaint problems then clear far faster.

Sources and last verified

Every provider claim on this page links to the source it came from, each with the date it was last checked. Provider policies change; if a detail looks off, the linked source is authoritative.

Related guides

Why is Outlook blocking my emails?Why are my emails going to spam in Gmail?Why is Yahoo blocking my emails?Why is iCloud blocking my emails?How to stop emails going to spam in OutlookWhy do emails go to junk in Hotmail and Outlook?Why does Outlook move emails to junk after they arrive?Why is Microsoft 365 blocking outbound emails?Why is Microsoft blocking my automated emails?550 5.7.26550 5.7.1421 4.7.0p=rejectinclude:adkim / aspf
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