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Why your emails land in spam — 12 checks that decide it

Inbox or spam is not luck. Here are the twelve signals mailbox providers weigh, and how to fix the ones working against you.

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When a campaign lands in spam, the instinct is to blame the subject line. Usually the real cause is further upstream. Mailbox providers score dozens of signals; here are the twelve that move the needle most — roughly in the order they are evaluated.

Authentication & identity

  1. SPF, DKIM, DMARC — missing or misaligned authentication is the fastest route to the spam folder. Fix these first.
  2. Reverse DNS (PTR) — your sending IP should resolve to a hostname that matches your HELO. Mismatches look like botnet traffic.
  3. Consistent From identity — switching From domains and friendly names erodes the reputation you have built.

Reputation

  1. IP and domain blocklists — a single listing on a major DNSBL can tank delivery. Check continuously, not once.
  2. Sending history — sudden volume spikes from a cold IP read as a spam run. Warm up.
  3. Engagement — opens, replies, and “not spam” clicks build reputation; spam complaints and deletes destroy it.

Content & structure

  1. HTML-to-text ratio — image-only emails with little text are a classic spam pattern. Always include a real plain-text part.
  2. Risky links — URL shorteners, mismatched anchor text, and links to low-reputation domains all add spam points.
  3. Spam-trigger phrases — “free”, “act now”, excessive punctuation and ALL CAPS still carry weight in content filters.
  4. Broken or bloated HTML — unclosed tags and 100KB+ messages get clipped and penalized.

Hygiene

  1. List quality — sending to spam traps and dead addresses signals poor list management. Use confirmed opt-in.
  2. Unsubscribe & headers — a working List-Unsubscribe header (and one-click variant) is now effectively required by Gmail and Yahoo for bulk senders.

Turn signals into a score

You do not have to guess which of these is hurting you. Send one message through DeliverSight and every check above comes back as a single score out of 10, with the specific failures ranked by impact — so you fix the one that matters before the eleven that do not.