"My email is going to spam" is the most common deliverability complaint, and it is almost never a mystery. Receiving mailboxes (Gmail, Microsoft, Yahoo) make their spam decision from a small set of testable signals. Most senders fail on the first one or two. Work through this list in order, because each cause gates the next: fixing your list quality will not help if your domain is not authenticated.
This is the most common cause and the fastest to fix. Receiving mailboxes want to see SPF, DKIM, and a published DMARC record on the sending domain. If any one is missing, or a record has a syntax error, the message is a documented spam candidate. A subtle version: the SPF record has a syntax error (a missing -all, a include: with no host) so it evaluates to a permanent error. The free DMARC checker and SPF checker surface both the missing-record and broken-record cases in seconds.
You can have all three records and still land in spam if the domains do not line up. The domain in the From header must align with the domain that passed SPF or DKIM. If you send From yourdomain.com but your ESP signs DKIM on esp-provider.com, DMARC alignment fails on your domain and the mailbox sees a failure. This is the second most common "I have everything but it still goes to spam" cause. The fix is to either send From the ESP's own domain or configure the ESP to sign with your domain.
Reputation is the history of how recipients have treated your mail: opens, replies, spam reports, and bounces, aggregated by the receiving provider. A brand-new IP or domain has no reputation and is treated as untrusted until it earns some. A previously-used IP or domain may carry complaints from a prior owner. If you recently changed ESPs or bought a "pre-warmed" IP, this is often the real cause. The fix is a slow ramp-up over days and, for a damaged domain, sometimes a fresh start.
High bounce rates and a high rate of user-reported spam are strong spam signals. Bounces happen when your list is full of dead addresses, role addresses that no longer exist, or addresses from domains that no longer accept mail. Complaints happen when people who never expected your mail mark it as spam. Practical levers: remove addresses that have not opened in 6+ months, stop buying lists, and make sure the content matches what the recipient signed up for. A sudden spike in spam reports is usually a list-quality problem, not a DNS problem.
Mailboxes also read the message itself. Triggers include a mismatch between the subject line and the body, too many images relative to text, tracking links that look like phishing, a missing or broken List-Unsubscribe header, and HTML that does not match the plain-text version. This layer matters most for commercial mail. The one-click unsubscribe header in particular is now a hard requirement for bulk senders at Gmail and Microsoft, and a broken one is a spam signal on its own.
New sending IPs and domains that suddenly send large volumes get sandboxed: the provider delivers a sample of your mail to evaluate it and routes the rest to spam or holds it. If you recently changed ESPs or started a campaign from a cold IP, the fix is a gradual ramp-up over days, not a full blast on day one. This is temporary by design, and it is the reason "it was fine until Tuesday" happens.
Your sending IP or domain may be on a blocklist, either because of a past abuse report or because a compromised account on your infrastructure sent spam. Being on a major blocklist can override everything else and force spam-folder delivery. Check your sending IP against the major public blocklists. If you are listed, request delisting and, more importantly, find and close the source that got you listed, or you will be relisted.
1. Authentication missing or broken? Run the free checker. Missing or malformed SPF, DKIM, or DMARC is the most common cause and the fastest to fix.
2. All three present but still spam? It is alignment, reputation, or list quality. Check the From domain against your DKIM signature, and look at your bounce and complaint rates in your ESP.
3. Recent change, sudden drop? Warmup, a broken record that recently expired, or a blocklist hit. Check the sending IP against the major lists and confirm your SPF still ends in -all.
Related: Transactional email deliverability · Cold email deliverability · Email not landing in Gmail · SPF vs DKIM vs DMARC · p=none vs quarantine vs reject · Is my domain blacklisted? · What is a good deliverability score?
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