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The Deliverability Ledger · Deliverability · 13 posts

What gets mail into the inbox

The signals inbox providers weigh, what each SMTP code and bounce type means, list hygiene, warm-up, and the numbers to measure yourself against.

All 13, newest first

  1. How to Check Email Deliverability: 5 Tests You Can Run TodayYou can't fix what you can't measure. Here are five free tests that give you a complete picture of your email deliverability in under an hour.8 min read
  2. How to Improve Email Deliverability: A 2026 PlaybookImproving deliverability comes down to a stack of small fixes. Here's the exact order to work through them, with what to measure at each step.11 min read
  3. SMTP Error Codes Explained: What 421, 550, 554 Really MeanEvery email rejection comes with an SMTP code. Most senders treat them as noise. Here's what each common code actually means, and how to fix it.9 min read
  4. Soft Bounce vs Hard Bounce: What They Mean and How to Fix ThemBounces are SMTP's way of telling you something failed. The difference between soft and hard matters enormously for reputation.7 min read
  5. What Affects Email Deliverability? 12 Signals Inbox Providers Care AboutInbox providers weigh 12 signals when deciding inbox vs spam. Here's the ranked list, with the ones you can actually move first.9 min read
  6. Email Deliverability Best Practices: The 2026 PlaybookA field-tested playbook of email deliverability best practices for 2026, covering authentication, warm-up, list hygiene, sender reputation, content, and inbox-placement monitoring.12 min read
  7. Email List Hygiene: The Complete Best-Practices GuideA dirty list is the fastest way to wreck sender reputation. Here's the verification, cleaning, and sunset routine that keeps your inbox placement healthy.9 min read
  8. Gmail & Yahoo Sender Requirements: 2026 Compliance GuideGmail and Yahoo's 2024 sender rules have escalated from warnings to outright rejection in 2026. Here's exactly what's required, and how to stay compliant.10 min read
  9. How to Warm Up an Email Domain: The 30-Day ScheduleA new sending domain has zero reputation. Send 10,000 emails on day one and you're in the spam folder forever. Here's the 30-day schedule that actually works.10 min read
  10. What Is a Good Email Deliverability Rate? Benchmarks for 2026The modern target is a 95%+ inbox placement rate. The 99% delivery rate your ESP reports measures something else. Here are the real 2026 benchmarks and how to measure yours.6 min read
  11. Microsoft’s May 2026 Defender Update: Why Cold Outreach Is Suddenly Hitting SpamIn May 2026, Microsoft quietly turned on a new generation of Defender filters. SMTP-based cold outreach is taking the hit, while Google Workspace and Microsoft 365 senders stay mostly untouched. Here's what changed, who's affected, and how to keep landing in the inbox.11 min read
  12. How Engagement, Complaints, and List Quality Affect Your Email DeliverabilityPerfect authentication and a clean IP aren't enough. Three behavioral factors (engagement, complaints, and list quality) determine whether you reach the inbox.12 min read
  13. Why Are My Emails Going to Spam? 7 Fixes That Actually WorkNearly half of all email traffic is classified as spam. Learn the 7 most common reasons your emails land in the spam folder, and exactly how to fix each one.10 min read

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