Why your emails land in spam — the deliverability checklist every sender needs

Why your email lands in spam

Authentication is the entry ticket; reputation is the inbox guarantee. A message with a clean DMARC pass, a valid SPF record, and a DKIM signature signed with your brand domain is allowed past the front gate — but it lands in the Primary tab only when the receiving mailbox provider trusts the sending domain. Trust is the slow accumulation of low bounce rates, low complaint rates, consistent volume, and authenticated infrastructure; it is lost fast (a single spamtrap hit, a single misaligned signing domain, a one-day burst at 50× your baseline volume can knock a healthy domain back into bulk). The rest of this checklist walks every lever you actually control, in the order they break first — authentication baseline, warm-up posture, AI-scored triage, list hygiene, then the recurring rollout that catches regressions before the receivers do.

The 2024 Gmail and Yahoo bulk-sender bar

Since February 2024, Gmail and Yahoo require any sender pushing more than ~5,000 messages per day to a Gmail or Yahoo inbox to satisfy a tight set of prerequisites or face throttling at Gmail and outright rejection at Yahoo. The bar is: a published DMARC record at p=none or stricter, an SPF record listing every sending IP and ESP, a DKIM signature on every outbound message using a domain that aligns with the visible From: header, a working one-click unsubscribe header (RFC 8058) on every marketing message, a spam complaint rate held below 0.1% as measured in Gmail Postmaster Tools, and a forward-confirmed reverse DNS (PTR) entry on the sending IP. A misconfigured ESP that signs with its own domain instead of yours is the most common failure — a passing DKIM check still fails DMARC alignment, and the receivers downgrade you. Run the SPF / DKIM / DMARC checker against your brand domain before any volume change and resolve every warn-tag the audit returns.

• DMARC Record Guide →
• SPF / DKIM / DMARC checker →

Warm-up posture for a brand-new domain

The first four weeks on a brand-new or re-activated sending domain decide whether your sends scale or stall. Mailbox providers have no reputation data on a new IP+domain pair and they throttle conservatively — a 50,000-message send on day one lands most of those messages in spam. The right pattern is a structured ramp: a low baseline volume for week one, a doubling cadence every 3–5 days, a hard cap on per-recipient frequency, a fresh engaged-recipient seed list, and tight bounce and complaint targets every step of the way. Most production senders reach a stable warm-up state after three to four weeks at moderate volumes; cold-outreach teams ramp slower (six to eight weeks) because they send to less-engaged recipient pools. The free warm-up checker shows you the volume curve, the per-week target, and the per-day send count that keeps you below the throttle line for every major provider.

• Email Warm-Up Guide →
• Email warm-up checker →

AI-scored deliverability triage

Authentication audits and warm-up posture checks tell you whether the plumbing is correct. The next question — the one that decides whether you actually reach the inbox — is whether your domain and IP carry enough reputation today to pass the providers’ invisible thresholds. The free AI-scored deliverability checker reads your DNS posture, your published authentication records, your PTR setup, your blocklist status, and your observable sending signals, and returns a 0–100 score plus the three to five risk factors driving it. Re-score daily during a warm-up ramp; re-score again whenever a sending IP swap, a new ESP onboarding, or a noticeable inbox placement drop changes the picture. A falling score is the early warning — the gap between a score drop and a Postmaster Tools remediation window is the window you have to fix the cause before the receivers do.

• Deliverability score checker →

List hygiene and content signals

Authentication and reputation carry a clean domain to the inbox; list hygiene keeps it there. The bar across the largest providers is strict: bounce rate held below 2% (and trending downward), hard-bounce rate below 0.5%, spam-trap exposure kept at zero (sweep every list against a known-trap database before a send), and spam complaint rate held below 0.1% as reported in Postmaster Tools and Microsoft SNDS. Anything above those lines starts to drag the Postmaster reputation rating from High toward Medium, and a Medium rating is the staging area before throttling. Content signals matter, but less than the receiver-side heuristics suggest: a clean, authenticated, low-bounce send with plain-text-like content performs well; the same send with image-heavy HTML, link-shortener redirects, and a deceptive subject line is the spam-folder trigger most teams blame first.

The recurring rollout

The biggest gap between senders whose mail lands in Primary and senders whose mail lands in spam is not any single one of the levers above — it is whether the sender re-reads those same signals every week. A new ESP onboarding, a shared-pool IP swap, a marketing subdomain drift, or a third-party signup flow that begins sending from your brand domain can each break a previously clean configuration in a single day. Set a weekly checklist: re-run the SPF / DKIM / DMARC audit, re-run the deliverability score, re-read your RUA aggregate feed for alignment regressions, and re-check your bounce and complaint rates against the provider-published bars. Catch the regression on day one and the fix is an afternoon; catch it on day thirty and you are rebuilding reputation from scratch.