An email landing in the spam folder is almost never down to a single cause. Gmail's, Outlook's, and Yahoo's filters don't apply one single rule: they add up signals. Every weak signal costs you points, and once the score drops below a certain threshold, your campaign goes to the wrong folder — or doesn't get delivered at all.
The good news is that most of those signals are within your control. Here are the 12 most common reasons, ranked by impact, separating what you can fix tomorrow from what's structural.
There are no guarantees in deliverability: no serious provider can promise you the inbox. What you can do is work every signal so the odds keep tipping consistently in your favor.
The filter isn't a gatekeeper anymore — it's an algorithm that learns
Spam used to be binary: you landed in the inbox or you fell into the junk folder. Today major inboxes use machine learning models, and that shifted the rules in two ways worth understanding before we get to the list.
First, delivery stopped being black or white. An email can reach the inbox but still get sidelined: under the Promotions tab, without a notification, or ranked below emails the inbox considers more relevant. Getting delivered is no longer enough — you're competing for visibility.
Second, filters increasingly weigh how people behave with your emails (whether they open, reply, archive, or delete without opening) and how clear the content is. On top of that, several email clients now show AI-generated summaries before the user opens the message: if your subject line and first lines don't say something clear, that summary won't either.
With that in mind, these 12 reasons stop being a technical checklist and become what the algorithm actually looks at to decide where to put you.
Authentication: making sure inboxes know it's really you
Without authentication, an inbox has no way to tell your domain apart from someone impersonating you. It's the foundation everything else rests on — and since 2024, Gmail and Yahoo require it from anyone sending at volume.
- You don't have SPF, or it's misconfigured. The SPF record declares which servers can send on behalf of your domain. If it's missing, or if you use
+all(which authorizes anyone), filters get suspicious. - You're missing DKIM. The DKIM signature guarantees the message wasn't tampered with in transit and that it came from an authorized sender. Without it, you lose one of the strongest identity signals available.
- You haven't published DMARC. DMARC tells inboxes what to do when SPF or DKIM fail, and sends you reports on who's sending mail as you. On top of that, a mismatch between the visible "From" domain and the domain that signs with DKIM sets off filters.
The fastest way to check where you stand is running the domain reputation diagnostic: it shows your SPF, DKIM, and DMARC status in seconds, no data required.
Reputation: your sending history carries weight
Inboxes remember how you behaved before. A domain or IP with a bad history drags that baggage into every new campaign.
- A new domain that hasn't been warmed up. Sending 50,000 emails from a domain that didn't exist yesterday is the fastest recipe for landing in spam. Reputation gets built gradually — that's exactly the point of warming up a domain over the first 30 days.
- Sharp volume spikes. Jumping from 1,000 to 100,000 sends overnight looks like spammer behavior. Ramp up volume gradually and predictably: consistency is itself a positive signal.
- Purchased or scraped lists. Besides being a bad practice, they trigger complaints and bounces that destroy your reputation. If they didn't ask to hear from you, don't add them.
Content and message format
The what and the how also count. Filters look for patterns historically tied to unwanted email — and, increasingly, how legible your message is to an algorithm summarizing it.
- Unbalanced text-to-image ratio. An email that's a single giant image with no text is suspicious (and looks broken if images don't load). Balance image and real text.
- Trigger-word subject lines and body copy. Sustained caps, strings of exclamation points, and exaggerated promises raise your spam score. Write the way you'd talk to a customer, not like a sale banner — and put what matters in the first lines: that's what both the rushed reader and the automatic summary pick up on.
- Broken HTML or suspicious links. Poorly closed code, generic URL shorteners, and link domains that don't match your sender domain erode trust. Keep your HTML clean, your structure clear (subheadings, short paragraphs), and your links on your own domain.
Subscriber behavior
This is the signal that's grown the most: AI-powered filters care, above all, about how people react to your emails.
- Sustained low engagement. If nobody opens or clicks for weeks, filters assume your mail isn't wanted and start sidelining you. Segment and send less, but better: prioritize the people who actually engage.
- "This is spam" marks. A single complaint carries enormous weight. Make unsubscribing easy (a visible link) so people choose to leave instead of reporting you.
- Lists without hygiene. Nonexistent addresses generate hard bounces that hit your reputation. Clean out bounces and spam traps on an ongoing basis.
The numbers worth watching
Major inboxes publish sender health guidelines (like Google's Postmaster Tools). They're not promises of inbox placement, but they are thresholds worth staying under:
| Signal | Directional target | Why it matters |
|---|---|---|
| Spam complaints | Below 0.1–0.3% | It's the signal that fastest sends you to the junk folder. |
| Hard bounces | Below ~2% | Lots of invalid addresses give away a dirty or purchased list. |
| Engagement (opens/clicks) | Sustained over time | It's what AI-powered filters weigh most heavily to decide your priority. |
| Authentication | 100% (SPF + DKIM + DMARC) | Without this, nothing else even gets evaluated in your favor. |
What you can do this week
You don't need to fix all 12 at once. In order of impact:
- Publish SPF, DKIM, and DMARC — follow the authentication step-by-step guide.
- Run the reputation diagnostic and note anything that shows up yellow or red.
- Remove hard bounces and contacts who never engaged from your list.
- Review your next send: a subject line that doesn't shout, what matters up in the first lines, a balanced image-to-text ratio, and a visible unsubscribe link.
How arrobaMail handles it
We handle a good part of this work on the infrastructure side. Our own sending engine, aMailMTA, manages retries, delivery pace, and sending IP health so your reputation doesn't depend on a noisy neighbor. Your Health Score sums up how you're doing in a single number, and before every send Amanda IA runs a pre-flight check that catches weak signals — from authentication to content — and suggests what to adjust.
None of this replaces good practices on your end: deliverability is a shared responsibility and an ongoing process, not a setting you configure once. But with the technical foundation handled, your work concentrates on what really moves the needle: a healthy list and a message people actually want to open.
Want to see where your domain stands today? Start with the free diagnostic or create a free account and try Amanda's pre-flight analysis on your next campaign.