Spam placement is a verdict, not a setting
When your mail lands in the spam folder, a receiving provider has weighed it and decided it is more likely unwanted than wanted. There is no single switch that caused it and no single switch that fixes it. The decision draws on a handful of signals, and the practical work is figuring out which signal is dragging you down rather than changing everything at once.
The causes below are roughly in the order a provider evaluates them, and roughly in the order worth checking. The first two are technical and you can confirm them today. The rest are about reputation and behavior, and they move over weeks rather than minutes.
Authentication and alignment
The first gate is whether the message proves who sent it. A receiver checks SPF and DKIM, and it checks DMARC alignment: the domain in your visible From header has to match either the SPF domain or the DKIM domain. Mail that fails this looks forged, and forged-looking mail is filtered or rejected outright.
Two subtleties catch people. You need SPF and DKIM both passing, not whichever was easier to set up. And you can pass both and still fail DMARC if neither aligns with your From domain, which is the usual reason "but SPF and DKIM are green" mail still goes to spam. If any of that is shaky, start with email authentication explained for how the three records fit together, then fix the specific failure before looking anywhere else. Authentication will not earn you the inbox on its own, but failing it guarantees you miss it.
Sender reputation
Once a message authenticates, the provider asks whether it trusts the sender. It scores your sending domain and IP on their own history: how much you send, how steadily, how often recipients want it, and how often they complain. Good reputation is the single strongest pull toward the inbox, and a poor or unknown one outweighs clean records.
A new domain or IP has no history, so early mail is treated cautiously until a pattern forms. Sudden volume from a cold sender resembles a spam run and gets filtered accordingly. The cure is gradual warm-up: small volumes to people who actually open your mail, correct authentication from the first send, and patience while a few weeks of good sending builds a record. Reputation is also why the same record set delivers for an established sender and fails for a brand-new one.
Engagement, complaints, and list quality
Reputation is largely a summary of how recipients react. Opens, replies, and saves push you toward the inbox; deletes-without-reading and especially the spam button push the other way. Gmail and Yahoo draw a hard line on the spam-complaint rate: keep it under 0.10% and never let it reach 0.30%, which works out to about three complaints per thousand delivered messages. Cross it and providers throttle or divert your mail no matter how it is signed.
Most complaint problems are really consent and relevance problems. Mailing people who never asked, keeping unengaged addresses on the list for years, or buying lists all produce complaints and spam-trap hits that wreck reputation. The bulk-sender side of this (the thresholds, one-click unsubscribe, and the complaint ceiling) is covered in Gmail and Yahoo bulk sender requirements.
Content and formatting
Content rarely sinks an established sender on its own, but it tips marginal cases. Filters react to image-only messages with almost no text, bulky or broken HTML, links pointing at domains with poor reputation, subject lines that misrepresent the body, and bulk mail with no working unsubscribe. Format to the RFC 5322 standard with a single valid From address, keep a sensible text-to-image balance, and make sure every link goes somewhere reputable. For a weak sender, clean content is the difference; for a strong one, it is insurance.
Infrastructure
A few baseline requirements sit underneath everything. The sending IP needs valid forward and reverse DNS, with a PTR record that is specific to your host rather than a generic provider default, and mail has to travel over TLS. If you send through a managed platform these are usually handled for you. If you run your own outbound server, they are yours to get right, and a missing PTR or a shared IP with bad neighbors can hold back otherwise clean mail.
When it is one provider and not the rest
If your mail reaches most inboxes but one provider buries it, the problem is usually specific to that provider's reputation system rather than a broken record. Each filter keeps its own history and its own rules, so the fix differs by mailbox.
For Gmail specifically, work through why Gmail rejects emails; the two cases that overlap most with this page are mail from an established domain that suddenly starts going to Gmail spam and a new domain whose mail goes to spam while reputation is still forming. For Microsoft and Outlook.com, why Outlook blocks emails covers SmartScreen, the Spam Confidence Level, and the SNDS and JMRP feedback that drive their decisions.
How to find your actual cause
Guessing wastes days. Test the real message instead. A deliverability test sends from your system and reads the result: it confirms SPF, DKIM, DMARC, and alignment from the Authentication-Results header, checks whether your domain or IP sits on a blacklist, and flags content that filters dislike. That separates the technical causes you can fix now from the reputation causes you manage over time. For the broader method and the tools involved, see how to test email deliverability and email deliverability tools.
For a fast first pass on the authentication and DNS side, run your domain through the instant snapshot on the home page. It surfaces missing SPF, DKIM, DMARC, reverse DNS, and TLS in one check, which covers the technical half of this list before you send anything.