The DeliverabilityLedger
Bounce codes decoded, provider policy changes tracked, authentication explained. Written for people whose mail has to arrive.
Every entry
SURBL Declares War on Cold Email: Why 90% of Domains We Tested Are Already Blacklisted
We 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.
11 Aug 202610 min37Google421 4.7.28 "Our system has detected an unusual rate of unsolicited mail originating from your IP address" — Gmail Fix Guide
Gmail's 421 4.7.28 is a temporary IP-reputation throttle — not a blocklist and not an authentication failure. Here's exactly what each piece of the bounce means and the step-by-step fix.
29 Jun 20268 min36Yahoo554 5.7.9 "Message not accepted for policy reasons" — Yahoo/AOL Fix Guide
Yahoo's 554 5.7.9 is a hard, policy-based rejection — not a temporary deferral. Here's what triggers it, why it's different from TSS04, and how to clear it.
27 Jun 20268 min35Yahoo421 4.7.0 [TSS04] "Messages temporarily deferred due to user complaints" — Yahoo Fix Guide
Yahoo's 421 4.7.0 [TSS04] response is a temporary, complaint-rate-driven deferral — not a blocklist. Here's what each part of the bounce means and the exact sequence to clear it.
26 Jun 20268 min34Google421 4.7.0 "Try again later, closing connection" — Gmail Rate Limit Decoded
Gmail'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.
12 Jun 20268 min33Google550 5.7.26 "This message does not have authentication information" — Gmail Fix Guide
Gmail'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.
12 Jun 20269 min32GoogleEmails Going to Spam in Gmail? The 2026 Recovery Playbook
If 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.
12 Jun 202610 min31GoogleGmail & Yahoo Bulk Sender Requirements: Complete 2026 Compliance Guide
Since 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.
12 Jun 202610 min30GoogleGoogle Postmaster Tools: The Complete Guide for Senders in 2026
Google 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.
12 Jun 202611 min29Microsoft421 4.7.500 "Server busy. Please try again later" — Microsoft Throttling Decoded
Microsoft's 421 4.7.500 / 4.7.501 deferrals usually aren't about server load — they're soft throttling tied to reputation. Here's how to spot the difference and recover delivery.
09 Jun 20267 min28Microsoft550 5.7.1 "Unfortunately, messages from [IP] weren't sent" — How to Fix It
The 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.
09 Jun 20269 min27Microsoft550 5.7.708 "Access denied, traffic not accepted from this IP" — Decoded
5.7.708 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.
09 Jun 20267 min26MicrosoftExchange Online Protection (EOP) Filtering Explained: SCL, BCL, and PCL Scores
Every 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.
09 Jun 20268 min25MicrosoftMicrosoft's Bulk Sender Requirements: What Changed and What You Need to Do
Microsoft'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.
09 Jun 20268 min24MicrosoftHow to Get Delisted from the Microsoft Outlook Blacklist (Step-by-Step)
Microsoft's delisting form looks simple but the approval rate plummets if you skip the prep work. Here's exactly what to fill in, what to attach, and what to do if you're denied.
09 Jun 20268 min23MicrosoftS3140 and S3150 Microsoft SMTP Errors Explained
S3140 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.
09 Jun 20267 min22MicrosoftMicrosoft SNDS: Reading the Data Like a Pro
SNDS 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.
09 Jun 20267 min21MicrosoftMicrosoft SNDS & JMRP: The Free Tools Every Sender Should Be Using
If 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.
09 Jun 20268 min20MicrosoftWhy 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.
09 Jun 202610 min19AuthenticationDMARC 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.
06 Jun 20268 min18DeliverabilityHow to Check Email Deliverability: 5 Tests You Can Run Today
You 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.
06 Jun 20268 min17DeliverabilityHow to Improve Email Deliverability: A 2026 Playbook
Improving deliverability isn't one trick — it's a stack of small fixes. Here's the exact order to work through them, with what to measure at each step.
06 Jun 202611 min16DeliverabilitySMTP Error Codes Explained: What 421, 550, 554 Really Mean
Every 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.
06 Jun 20269 min15DeliverabilitySoft Bounce vs Hard Bounce: What They Mean and How to Fix Them
Bounces are SMTP's way of telling you something failed — but the difference between soft and hard matters enormously for reputation.
06 Jun 20267 min14DeliverabilityWhat Affects Email Deliverability? 12 Signals Inbox Providers Care About
Inbox providers weigh 12 signals when deciding inbox vs spam. Here's the ranked list, with the ones you can actually move first.
06 Jun 20269 min13DeliverabilityEmail Deliverability Best Practices: The 2026 Playbook
A field-tested playbook of email deliverability best practices for 2026 — covering authentication, warm-up, list hygiene, sender reputation, content, and inbox-placement monitoring.
05 Jun 202612 min12DeliverabilityEmail List Hygiene: The Complete Best-Practices Guide
A dirty list is the fastest way to wreck sender reputation. Here's the verification, cleaning, and sunset routine that keeps your inbox placement healthy.
05 Jun 20269 min11DeliverabilityGmail & Yahoo Sender Requirements: 2026 Compliance Guide
Gmail 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.
05 Jun 202610 min10DeliverabilityHow to Warm Up an Email Domain: The 30-Day Schedule
A 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.
05 Jun 202610 min09DeliverabilityWhat Is a Good Email Deliverability Rate? Benchmarks for 2026
A 95%+ inbox placement rate is the modern target — not the 99% delivery rate your ESP reports. Here are the real 2026 benchmarks and how to measure yours.
05 Jun 20266 min08BlacklistsIP Blacklists vs. Domain Blacklists: Why You're Probably Checking the Wrong One
Most 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.
03 Jun 202612 min07DeliverabilityMicrosoft's May 2026 Defender Update: Why Cold Outreach Is Suddenly Hitting Spam
In 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.
30 May 202611 min06DeliverabilityHow Engagement, Complaints, and List Quality Affect Your Email Deliverability
Perfect authentication and a clean IP aren't enough. Three behavioral factors — engagement, complaints, and list quality — determine whether you reach the inbox.
02 Apr 202612 min05Cold EmailHow to Improve Inbox Placement for Cold Email (Without Getting Blacklisted)
Cold email deliverability in 2026 is harder than ever. Here's how to actually land in the primary inbox — infrastructure, content, and monitoring strategies that work.
25 Mar 20269 min04BlacklistsEmail 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.
18 Mar 20267 min03ReputationWhy Did My Email Sender Reputation Drop? 7 Causes and How to Recover
Your email sender reputation is the single most important factor determining whether your messages reach the inbox. Here's what causes it to drop and how to recover.
10 Mar 202611 min02AuthenticationHow to Set Up and Fix SPF, DKIM, and DMARC: A Step-by-Step Guide
Learning 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.
03 Mar 202615 min01DeliverabilityWhy Are My Emails Going to Spam? 7 Fixes That Actually Work
Nearly 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.
24 Feb 202610 minStop reading. Start checking.
Every fix in these posts starts with knowing what is actually wrong. Run the full scan on your domain — free, no signup.