You sent the same abandoned-cart flow yesterday and it performed. Today the replies say “it went to spam,” and the ESP still shows the messages as delivered.
That gap is the whole problem. Delivered means a receiving server accepted the message. Spam is a later decision, made per mailbox provider, and your campaign report does not show it. This is the diagnosis for mail that was landing, then suddenly was not. If your mail has been in spam from the start, or you want the short operational checklist, use emails going to spam. The rest of this page is how to tell the causes of a sudden move apart.
A sudden move to spam almost always follows something that changed in your own sending: a new source, a colder audience, or more volume. That is a working pattern, not a provider rule. Filters also change on the provider side, which is why you confirm the cause on a real message before you fix anything.
Last verified: 26 September 2026, against Google’s sender guidelines, Yahoo Sender Hub, and Microsoft’s documentation for 550 5.7.515.
What is actually happening
Three different outcomes get called “spam” in a Slack thread:
| What you see | What it means | Where to look |
|---|---|---|
| ESP says delivered, subscriber finds it in spam | The mailbox accepted the message, then filtered it | Placement at that provider, complaint rate, engagement |
ESP says bounced, with 550 5.7.515 |
Outlook.com refused it. It never arrived | Authentication on the visible From domain |
| ESP says delivered, subscriber sees nothing | It may be in Promotions, a filter, or another account | Delivered but not received |
Inbox placement and delivery rate are different measurements. Fixing the wrong one wastes the day. When only one provider moved, follow why placement differs by provider.
Why your emails are suddenly going to spam
Mailbox providers do not publish the full filter. They do publish requirements, and those requirements explain most sudden failures when your mail was previously fine.
Authentication stopped matching the From address. Gmail requires every sender to use SPF or DKIM. Senders of close to 5,000 or more messages to personal Gmail accounts within 24 hours must use SPF and DKIM, publish DMARC at p=none or stronger, and align the From domain with SPF or DKIM. Yahoo’s bulk-sender rules are the same shape: SPF and DKIM, a DMARC policy of at least p=none, and DMARC must pass. Microsoft documents a separate failure: since May 5, 2025, domains sending more than 5,000 messages a day to Outlook.com, Hotmail.com, or Live.com must pass SPF, DKIM, and DMARC (at least p=none, aligned). Non-compliant mail goes to Junk first, with rejection (Outlook 550 5.7.515) following later.
Your new Klaviyo flow, a review app, or a transactional tool that sends as @yourstore.com without being in SPF or DKIM will fail this check on the next campaign even if last month was clean.
Complaint rate moved. Google’s guideline is to keep the Postmaster Tools spam rate below 0.10% and to avoid ever reaching 0.30%. That 0.10% figure is the level Google tells you to stay under. The 0.30% figure is the point at which Google says bulk senders become ineligible for mitigation, until the rate stays below 0.30% for seven consecutive days. Yahoo tells bulk senders to keep the spam rate below 0.30%, and says that rate is calculated on mail delivered to the inbox, not on every attempted send. Neither number is a promise that mail under the line reaches the inbox.
Postmaster Tools measures user-reported spam on DKIM-authenticated mail that reached an engaged recipient’s inbox and was then marked spam. Your ESP’s complaint rate is a different dataset. Klaviyo says Gmail does not provide spam-complaint feedback to ESPs, so a calm Klaviyo complaint chart can sit next to a bad Gmail spam rate. Read what 0.10% and 0.30% mean.
The audience got colder. A win-back, a “whole list” sale, or a welcome series that keeps emailing people who never clicked will raise complaints and lower the engagement signals providers use after authentication. The creative did not have to change. Welcome series deliverability and list hygiene are the usual places this starts.
Volume or infrastructure changed. A dedicated IP with no history, a subdomain that has never sent, or a shared pool you just joined does not inherit last quarter’s placement. IP reputation and domain reputation move on different clocks.
Passing authentication does not guarantee the inbox. A dedicated IP does not guarantee it either. A low bounce rate does not guarantee it.
Spam folder diagnostic checklist
The symptom narrows the search before you open a header. One campaign in spam and the next in the inbox points at audience and content, not DNS. Every Gmail address in spam while Yahoo still inboxes points at Postmaster Tools compliance and spam rate, then the Gmail troubleshooting guide. Outlook addresses hard-bouncing while everyone else is fine means you read the SMTP code before you touch the list. Flows in spam with campaigns fine, or the reverse, usually means the streams use different domains, different complaint rates, or overlapping sends, which Klaviyo flows versus campaigns covers. And if nothing in your ESP looks wrong while customers still screenshot spam, you are looking at placement, which the ESP does not report: how to check where mail landed.
Then work in this order. Stop when the evidence is specific.
- Pull one raw message from the spam folder and read
Authentication-Results. - Confirm the visible From domain, the DKIM
d=domain, and the envelope sender. Alignment fails when those domains are unrelated, which is common when you send marketing from a subdomain and transactional mail from the root, or the reverse, without DMARC set up for both. - In Google Postmaster Tools, read Compliance status and the spam-rate dashboard for the domain that actually signed the mail. Google has said the Domain and IP Reputation dashboards will not move to Postmaster Tools v2 and will be retired. Do not build your monitoring around them; use v2’s Compliance Status and spam rate instead.
- For Yahoo and AOL, check Yahoo and AOL seed-mailbox placement and Yahoo’s own Sender Hub, and whether the DKIM domain is enrolled in Yahoo’s complaint feedback loop.
- Compare the failing send with the last healthy one: list or segment, size, offer, and whether a flow fired the same day.
- Check bounce codes. A
5xxrejection is not a spam-folder problem. A4xxdeferral is not a spam-folder problem either. - Only then look at content: image-only mail, broken links, or a from-name the subscriber has never seen.
The short version of the same checks is on the fix page.
What to fix, by cause
Authentication. Add the new sender to SPF without exceeding the lookup limit, turn DKIM on for that domain, and publish DMARC if it is missing. SPF, DKIM, and DMARC is the setup reference. No DMARC record is the Klaviyo and Shopify case. Re-send only after a live message shows pass, not after the DNS panel looks right.
Complaints. Stop the send that is producing them. That is usually a cold segment, a second email the same day, or a win-back. Suppress complainers. Make the unsubscribe obvious. Google and Yahoo both require one-click unsubscribe on bulk marketing mail, and Google recommends honoring requests within 48 hours. Neither, by itself, repairs a rate that is already high.
List quality. Hard bounces should already be suppressed by the ESP. A spike in bounces means a bad import, a typo-prone popup, or a list you bought. Do not mail that source again while you investigate. Spam traps are the expensive version of the same mistake.
A single provider. Follow the provider guide rather than applying a generic “clean the list” pass to a Gmail-only or Yahoo-only failure. Outlook rejections are authentication on the From domain. Yahoo complaint handling is covered in Yahoo email deliverability.
What not to do while you fix it. Do not rewrite the subject line first. Content matters, and InboxEagle’s own studies on spam-trigger words and sale subject lines are the place to test that later, but it is a poor first move when authentication or complaints changed. Do not buy a list of “engaged” addresses to dilute the complaint rate. Do not remove the unsubscribe link: people who cannot leave use the spam button, and that is the signal Gmail and Yahoo count. And do not read open rate as proof of inbox placement, because open rate is a weak signal once Apple Mail Privacy Protection is in the mix.
How to verify the fix
Send the next message to people who clicked recently, not to the full list. On that send, read the authentication results on the received message, not only in the DNS tool. Check placement at Gmail, Yahoo, AOL and Outlook: InboxEagle’s seed list uses real mailboxes at those four providers and reports inbox, spam, or promotions. An Outlook 550 is a rejection at SMTP time, not a folder placement, so it still has to be read from your bounce log. Then watch Gmail spam rate in Postmaster Tools over the following days. One clean seed test does not reset a 0.30% spam rate. Google’s mitigation rule is seven consecutive days under 0.30% before a bulk sender is eligible again, and that is about mitigation eligibility, not a promise of inbox placement.
If placement is still spam after authentication passes and the audience is your recent clickers, treat it as a reputation problem and move to the recovery process. Do not launch a new domain in the same afternoon; changing your sending domain explains what a new domain does and does not reset.
When to escalate, and what to watch after
Escalate to your ESP when authentication passes on mail you send, the engaged segment still lands in spam at one provider, and you can show the dates and the spam rate. Ask about the shared IP or the domain reputation on their side, and include the bounce text if Outlook is rejecting.
Escalate internally, and pause nonessential promotions, when Postmaster Tools spam rate is at or above 0.30%, or when several campaigns in a row are in spam at more than one provider. That is a first-24-hours recovery problem, not a creative test.
Once you are back in the inbox, check placement on the sends that make the revenue: campaigns, and the flows that mail the same person in the same day (welcome, browse, cart, post-purchase). Watch Gmail spam rate in Postmaster Tools, Yahoo complaints in Sender Hub, and authentication failures per campaign. A weekly glance at open rate will not catch a provider-specific spam placement until the revenue has already moved.
InboxEagle
Rule out authentication and blocklists first
The free deliverability checker grades SPF, DKIM, DMARC, blocklists, and MX on your sending domain. If it comes back clean, the sudden spam move is audience or reputation, and seed testing shows the folder.
Sources
- Email sender guidelines, Google. Bulk-sender definition, authentication, spam-rate guidance.
- Email sender guidelines FAQ, Google. What happens above a 0.30% spam rate, including the seven-day mitigation window.
- Postmaster Tools dashboards, Google. How spam rate is calculated.
- Sender requirements, Yahoo Sender Hub. Bulk-sender authentication and the 0.30% complaint rate.
- Fix NDR 550 5.7.515, Microsoft. High-volume sender definition and the rejection.
- Email deliverability FAQs, Klaviyo. Gmail complaint data is not returned to the ESP.