Nothing changed. Same newsletter template, same sending tool, same Tuesday morning slot. And yet the open rate halved, and a colleague forwarded a screenshot of your campaign sitting in their Gmail spam folder.
It is tempting to blame the subject line, or the new image, or a word that sounds salesy. Usually it is none of those. Your domain did not get bad at email. It got judged, and the verdict applies to everything it sends.
Spam placement is a verdict on the sender
Gmail, Outlook and Yahoo decide where a message goes mostly by asking one question: do we trust whoever sent this? The answer comes from months of history attached to your sending domain and the IP addresses that send for it.
That is why a perfectly ordinary message can land in spam, and why moving one campaign around does not fix it. The filter is not reading the email in isolation. It is reading your record.
The five things mailbox providers actually judge
1. Authentication. Can the provider confirm the mail really comes from your domain? That is what SPF, DKIM and DMARC are for. Large senders must have all three: Gmail and Yahoo require it for bulk senders, and Gmail moved to rejecting non-compliant mail outright rather than only filtering it. Microsoft rejects failing high-volume mail to Outlook addresses with a 550 5.7.515 error.
2. Complaints. Every "Report spam" click counts against you. Google asks bulk senders to keep the rate reported in Postmaster Tools below 0.1% and never to reach 0.3%. That is one complaint in a thousand messages.
3. Bounces. Mail to addresses that do not exist tells providers you do not know your audience. Why bounces burn more than a campaign explains how a few hundred dead addresses spread into everything else.
4. Volume patterns. A domain that sends 500 emails a week and then 50,000 on a Tuesday looks compromised or careless. Sudden spikes invite suspicion even when the list is clean.
5. Engagement. Opens, replies, moving mail out of spam, adding you to contacts: positive signals raise trust. Mail that is ignored, deleted unread or left in spam lowers it.
The part content plays
Content matters less than most people think, but it is not irrelevant. Misleading subject lines, link shorteners, a single image with no text and links to domains with poor reputations all add weight on the wrong side.
On a domain with a strong record, ordinary marketing copy rarely decides anything. On a domain already under suspicion, it can be the last straw. Fix the record first.
Four explanations that waste a week
Before rewriting the newsletter, rule these out. They are the usual suspects, and they are usually innocent.
- "A word in the subject line triggered a filter." Modern filters weigh sender history far above vocabulary. The word
freein a subject line from a trusted domain is fine. The same word from a domain with a 4% bounce rate is not the reason it was filtered. - "Our image to text ratio is wrong." A single large image with two lines of text is a weak signal, not a verdict. It matters at the margin, on a sender who is already borderline.
- "The unsubscribe link makes us look like a marketer." The opposite. A visible, working unsubscribe link is required for bulk senders and prevents the complaints that actually hurt.
- "We should switch sending platform." Domain reputation travels with the
Fromdomain, not with the tool. A new platform changes the IP and nothing else, and on a shared pool it can make things worse.
The common thread: content decides edge cases, reputation decides the rest.
Spam folder, or never delivered at all?
These look the same from a sender's chair and need opposite fixes.
Filtered means the message was accepted and then placed in spam. Your platform reports it as delivered, the open rate collapses, and someone eventually finds it in a spam folder.
Blocked means the receiving server refused it. It appears in your bounce log with a 5.7.x code and wording about policy, reputation or a blocklist. Nobody will ever find it, because it was never stored.
| Symptom | Likely state | Next step |
|---|---|---|
| Delivered, near-zero opens at one provider | Filtered | Check authentication and complaint rate |
550 5.7.1, 554 5.7.1 in the bounce log | Blocked by policy or reputation | Read the full text of the rejection, it often names the reason |
| The rejection mentions a blocklist URL | Listed | Check the domain reputation and IP reputation tools, then request delisting |
| Delivery delayed by hours, then arrives | Throttled | Reduce volume per hour and keep it steady |
How to find out which one it is
- Look at the headers of a message that landed in spam.
spf=pass,dkim=passanddmarc=passshould all be there. Afailis your first fix. - Check Google Postmaster Tools for your domain's spam rate and compliance status with Gmail's sender requirements.
- Check the blocklists. Run your domain through the domain reputation checker and your sending IP through the IP reputation checker. A listing explains sudden rejections.
- Read your bounce report per campaign and per list source. One source that bounces far more than the others is often the whole story.
Read the headers yourself
Every message carries the receiving provider's own verdict on your authentication. It takes a minute to read and settles most arguments.
In Gmail, open the message, choose "Show original" from the menu. In Outlook on the web, open the message properties. Look for the Authentication-Results line:
Authentication-Results: mx.google.com;
dkim=pass header.d=example.com;
spf=pass smtp.mailfrom=bounce.example.com;
dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=example.com
Three things have to be true:
dkim=pass, andheader.dis your own domain, not your sending platform's. A signature fromsendgrid.netpasses, but it does not build your reputation.spf=pass, for the domain in the envelope sender.dmarc=pass, which requires SPF or DKIM to be aligned with the domain your recipients actually see in theFromfield.
Send one test to a Gmail address and one to an Outlook.com address before any campaign goes out. If any line says fail, softfail, none or temperror, stop here: nothing else you change will be judged fairly until this reads pass.
Getting back to the inbox
Order matters:
- Fix authentication. Nothing else sticks while SPF, DKIM or DMARC fail.
- Stop sending to addresses you have not verified. Run your lists through verification and remove invalid addresses. How to reduce your email bounce rate covers the three moves.
- Send to your most engaged contacts first. People who opened or clicked recently rebuild trust fastest.
- Raise volume gradually over a few weeks instead of returning to full blasts on day one.
- Make unsubscribing effortless, so annoyed recipients leave instead of complaining.
The habit that prevents most of it
Most spam-folder stories start with a list nobody checked. Keep this short:
- Verify addresses when they are collected, so disposable and mistyped ones never enter. Disposable email detection shows the signup side.
- Verify any list older than a few months before sending to it.
- Suppress hard bounces and complaints automatically, everywhere.
- Watch spam rate and bounce rate every month, not only when something breaks.
The fastest way to protect a sender reputation is to stop giving providers reasons to doubt it. Bulk verification cleans the list you send to next.
