The Deliverability Ledger · 38 posts · real bounce strings
Bounce codes, decoded
Provider policy changes tracked, error codes explained, authentication made legible. Written for people whose mail has to arrive.
Newest
BlacklistsSURBL Declares War on Cold Email: Why 90% of Domains We Tested Are Already BlacklistedWe pulled live data on 2,679 active cold-email sending domains and checked every one against SURBL's Multi blacklist. 90% came back listed. The pattern behind who gets hit isn't what most senders assume.Google421 4.7.28 “Our system has detected an unusual rate of unsolicited mail originating from your IP address” — Gmail Fix GuideGmail's 421 4.7.28 is a temporary IP-reputation throttle, separate from blocklists and authentication failures. Here's exactly what each piece of the bounce means and the step-by-step fix.Yahoo554 5.7.9 “Message not accepted for policy reasons” — Yahoo/AOL Fix GuideYahoo's 554 5.7.9 is a hard, policy-based rejection rather than a temporary deferral. Here's what triggers it, why it's different from TSS04, and how to clear it.
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Deliverability
13 postsThe signals inbox providers weigh, what each SMTP code and bounce type means, list hygiene, warm-up, and the numbers to measure yourself against.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
Microsoft
10 postsThe 550 and 421 errors Microsoft returns, how Exchange Online Protection scores a message, SNDS and JMRP, and how to get off the Outlook blocklist.
- 421 4.7.500 “Server busy. Please try again later” — Microsoft Throttling DecodedMicrosoft's 421 4.7.500 / 4.7.501 deferrals usually mean soft throttling tied to reputation. Server load is rarely the cause. Here's how to spot the difference and recover delivery.
- 550 5.7.1 “Unfortunately, messages from [IP] weren’t sent” — How to Fix ItThe 550 5.7.1 rejection from Outlook.com almost always points at IP reputation. Here's how Microsoft decides to block, and the exact steps to get unblocked and stay that way.
- 550 5.7.708 “Access denied, traffic not accepted from this IP” — Decoded5.7.708 is the Microsoft 365 / Exchange Online cousin of S3140, but the cause is often a shared sending pool, not your domain. Here's how to diagnose and fix it.
- Exchange Online Protection (EOP) Filtering Explained: SCL, BCL, and PCL ScoresEvery message that hits a Microsoft 365 mailbox gets graded by EOP with SCL and BCL scores in the headers. Reading them turns junk-folder debugging from guesswork into evidence.
- Microsoft’s Bulk Sender Requirements: What Changed and What You Need to DoMicrosoft's bulk-sender requirements mirror Gmail and Yahoo, but with stricter enforcement timelines. If you send 5,000+ messages a day to Outlook addresses, this is the checklist.
- How to Get Delisted from the Microsoft Outlook Blacklist (Step-by-Step)Microsoft runs two delisting routes: one for Outlook.com and one for Microsoft 365. What to prepare first, what to fill in, and what to do if you are denied.
- S3140 and S3150 Microsoft SMTP Errors ExplainedS3140 means Microsoft's reputation engine has listed your IP. S3150 means it listed you *and* your authentication or content looks bad. Here's how to read the codes and clear them.
- Microsoft SNDS: Reading the Data Like a ProSNDS is the most honest reputation signal Microsoft gives you, if you know what each column means. Here's how to read the dashboard column by column and what to act on first.
- Microsoft SNDS & JMRP: The Free Tools Every Sender Should Be UsingIf you send any meaningful volume to Outlook.com or Hotmail, you should be in SNDS and JMRP. They're free, take 15 minutes to set up, and tell you what Microsoft thinks of your IPs in near real time.
- Why Your Emails Land in Outlook’s Junk Folder (and How to Fix It in 2026)Outlook's junk decision is a weighted sum of reputation, authentication, content, and engagement. Here's how to influence each one, and the order to fix them in.
The Gmail bounces senders hit most (421 4.7.0, 421 4.7.28, 550 5.7.26), the bulk sender requirements, Postmaster Tools, and the way back out of the spam folder.
- 421 4.7.28 “Our system has detected an unusual rate of unsolicited mail originating from your IP address” — Gmail Fix GuideGmail's 421 4.7.28 is a temporary IP-reputation throttle, separate from blocklists and authentication failures. Here's exactly what each piece of the bounce means and the step-by-step fix.
- 421 4.7.0 “Try again later, closing connection” — Gmail Rate Limit DecodedGmail's 421 4.7.0 is a temporary deferral, but the trigger varies. Here's how to tell whether it's volume, reputation, or per-connection limits, and how to fix each.
- 550 5.7.26 “This message does not have authentication information” — Gmail Fix GuideGmail's 550 5.7.26 bounce means your mail failed Gmail's bulk-sender authentication checks. Here's exactly which check is failing and the precise fix for each one.
- Emails Going to Spam in Gmail? The 2026 Recovery PlaybookIf Gmail is filing your emails into spam, the cause is one of seven specific things. Here's the diagnostic sequence and the 30-day recovery playbook.
- Gmail & Yahoo Bulk Sender Requirements: Complete 2026 Compliance GuideSince February 2024, Gmail and Yahoo enforce strict requirements on bulk senders. Here's the complete 2026 compliance checklist: authentication, unsubscribe, and the 0.3% spam ceiling.
- Google Postmaster Tools: The Complete Guide for Senders in 2026Google Postmaster Tools is the only place Gmail tells you what it actually thinks of your sending. Here's how to set it up, verify, and read every dashboard like a deliverability pro.
Blacklists
3 postsHow to tell if you are listed, why IP and domain blacklists catch different things, and what SURBL listings mean for cold email.
- SURBL Declares War on Cold Email: Why 90% of Domains We Tested Are Already BlacklistedWe pulled live data on 2,679 active cold-email sending domains and checked every one against SURBL's Multi blacklist. 90% came back listed. The pattern behind who gets hit isn't what most senders assume.
- IP Blacklists vs. Domain Blacklists: Why You’re Probably Checking the Wrong OneMost senders worried about blacklists are checking the wrong thing entirely: wrong IP, wrong list type, or both. Here's how IP and domain blacklists actually work, which ones matter for B2B cold email, and how to find the IP that really got you listed.
- Email Blacklist Check: How to Know If You’re Blacklisted (and How to Fix It)If your domain or sending IP is on a blacklist, your emails are being filtered or rejected. Here's how to check, which lists matter, and how to get delisted.
Authentication
2 postsSetting up SPF, DKIM and DMARC step by step, and choosing between a quarantine and a reject policy.
- DMARC Quarantine vs Reject: Which Policy to Use (and When to Switch)p=none, p=quarantine, p=reject: three settings, three very different outcomes. Here's exactly when to pick each, and the safe path to enforcement.
- How to Set Up and Fix SPF, DKIM, and DMARC: A Step-by-Step GuideLearning how to set up SPF, DKIM, and DMARC is simpler than those acronyms suggest. Most people can fix their email authentication in under an hour.
Yahoo
2 postsThe two refusals Yahoo and AOL send most, 421 [TSS04] for complaints and 554 5.7.9 for policy, and how to clear each.
- 554 5.7.9 “Message not accepted for policy reasons” — Yahoo/AOL Fix GuideYahoo's 554 5.7.9 is a hard, policy-based rejection rather than a temporary deferral. Here's what triggers it, why it's different from TSS04, and how to clear it.
- 421 4.7.0 [TSS04] “Messages temporarily deferred due to user complaints” — Yahoo Fix GuideYahoo's 421 4.7.0 [TSS04] response is a temporary, complaint-rate-driven deferral rather than a blocklist. Here's what each part of the bounce means and the exact sequence to clear it.
Cold Email
1 postGetting cold outreach into the inbox without burning the domain it is sent from.
Reputation
1 postThe usual reasons a sender reputation drops, and how to rebuild it.
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