Anatomy of a Deliverability Incident
Three compounding causes hid behind one Gmail crisis. A full root-cause walkthrough, action plan, and the trade-offs InboxStack Brain leaves for you to call.
Field notes
Email deliverability — what we've seen this month.
In-depth guides, audits, and post-mortems on inbox placement, authentication, and sender reputation. Written by the engineers running Brain — not a content team.
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The library
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Filter by topic or skim the full archive. New posts every few weeks.
Email Authentication
DMARC Alignment Failure: Why SPF and DKIM Can Both Pass and DMARC Still Fails
A message can pass SPF and pass DKIM individually and still fail DMARC. Alignment is the missing piece — what it checks, why it fails, and how to fix it.
4 min readEmail Deliverability
Why Your Emails Went to Spam: Finding the Cause, Not Just the Symptom
Your inbox rate dropped — but which of the five causes actually did it? A method for diagnosing why mail lands in spam, in the order that rules things out fastest, instead of guessing at fixes.
4 min readEmail Deliverability
Choosing an Email Tool for Agencies: What Breaks Once You Pass Ten Client Domains
Single-domain tools break down once you're managing 10+ client accounts. What actually matters: per-client isolation, delegated access, and reporting you can hand to a client without editing it first.
4 min readEmail Authentication
How to Fix an SPF Fail: Softfail, Hardfail & PermError Explained
SPF fail, softfail, and PermError all mean different things and need different fixes. The exact causes, how to tell them apart, and how to fix each one — with a free record generator.
5 min readEmail Deliverability
Multi-ESP Email Deliverability: What Breaks When You Send From More Than One Provider
Transactional through one ESP, marketing through another, cold outbound through a third — each fine alone. Together they cause SPF lookup overflow, DKIM misalignment, and split domain health. Here's the audit.
4 min readInbox Placement
Why Your Aggregate Inbox Rate Is Lying to You
One blended inbox placement number can hide a 30-point gap between Gmail and Outlook. What to expect at each provider and ESP, and how to read your own numbers against it.
9 min readEmail Authentication
BIMI Setup: Get Your Logo Next to Your Emails in Gmail and Yahoo
Your logo shows in Gmail and Yahoo only once DMARC is enforcing, the SVG passes the Tiny P/S check and a VMC is issued. What you need to qualify, the exact DNS record, and the trademark step that takes the longest.
8 min readEmail Deliverability
Your Mail Is Going to Spam: Which Tool Actually Fixes It
A warmup tool can't repair a domain Gmail has already scored badly, and a one-off seed test can't warm a cold one. Which of the five tool types fits your problem, volume and team, and the questions to ask before you pay.
9 min readInbox Placement
How to Test Where Your Email Actually Lands Before You Send
Your open rate just tanked and you don't know why. Here's how to run a seed test that shows exactly where each provider drops your email — before your subscribers find out first.
9 min readEmail Deliverability
GlockApps vs InboxStack vs InboxEagle: What Multi-Domain Senders Actually Need
Honest comparison of the three tools multi-domain senders evaluate most. When each one is the right choice — and when it isn't.
13 min readEmail Authentication
"My DMARC Says Pass But I'm Still in Spam" — Why This Happens
DMARC passing doesn't guarantee inbox placement. Here are the 7 real reasons your authenticated email still lands in spam — and exactly how to fix each one.
9 min readInbox Placement
Your 98% Delivery Rate Is Hiding a Spam Problem
A 98% delivery rate and a 60% inbox rate can be the same campaign. The difference between email delivery, email deliverability, and inbox placement — and which one your ESP dashboard isn't showing you.
7 min readEmail Authentication
Why Gmail Is Blocking Your Emails — And the 5-Minute Fix
If your emails are being rejected by Gmail or Yahoo, the cause is almost always authentication. Here's the exact 5-minute check — written by a 15-year ESP veteran.
11 min readEmail Authentication
Google's 2026 Sender Rules: What Breaks First When You Fail Them
The four requirements Gmail and Yahoo now enforce. What happens when you fail each one — and how to fix it in order.
8 min readInbox Placement
Inbox Placement: The Number Your ESP's 'Delivered' Rate Hides
Your ESP says 98% delivered. Up to 22% of that can be sitting in spam with no bounce to show for it. What inbox placement measures, how to calculate it, what Gmail, Outlook and Yahoo averages look like, and how to raise yours.
10 min readGeneral
Email Deliverability Audit: The Checklist That Finds Why a Client's Mail Is in Spam
A client's open rate halved and nobody knows why. The audit checklist that finds the cause — SPF, DKIM and DMARC, domain health, list, content and infrastructure — in the order that rules things out fastest.
8 min readGeneral
Your List Is Hurting Your Inbox Rate: How to Clean It Without Losing Real Subscribers
Bounce rate over 2% and complaints creeping toward Gmail's 0.10% line. How to find the addresses dragging your list down, run re-engagement before you suppress, and set a sunset policy so it stays clean.
9 min readGeneral
Your Sender Reputation Is Dropping. Here's How to See It First.
By the time Postmaster flags a compliance failure, you're already losing mail. Here's the earlier signal — and how to catch it.
13 min readGeneral
The 6-Week Warmup Plan That Doesn't Get a New Domain Blocked
Warm a new domain or IP too fast and Gmail defers you for weeks. A week-by-week volume curve, the segments to send to first, the numbers that mean stop, and what Gmail, Outlook and Yahoo each want.
9 min readGeneral
Gmail Promotions Tab: Why Your Emails Land There and How to Get Back to Primary
Gmail files your campaign under Promotions and opens drop 10-30%. The signals that push mail there, the ones that pull it back to Primary, the five moves that actually work, and the tricks that backfire.
8 min readEmail Deliverability
How Machine Learning Catches a Spam-Folder Drop Before Your Dashboard Does
Gmail changes its filters weekly and a manual check once a day misses the drop until the damage is done. How anomaly models spot a complaint spike or a broken DKIM key within hours, and what that looks like in practice.
4 min readEmail Deliverability
ESPs: How to Know a Customer's Mail Is in Spam Before They File the Ticket
Five hundred customer domains, each one able to hit a blocklist or fail DKIM on any given day. How ESPs watch every domain at once, catch a customer's spam spike before the support ticket, and stop churn that starts in the inbox.
4 min readEmail Deliverability
Your IP Is on a Blacklist: How to Check, Get Delisted and Stay Off
A Spamhaus listing can block a third of your mail overnight. How to check every major blocklist for your IP and domain, get delisted without being denied, and stop the problem that put you there from coming back.
4 min readEmail Deliverability
Your Inbox Rate Is Slipping: The 12 Fixes, Ranked by How Fast They Pay Off
Opens are down and nothing in your ESP dashboard explains it. Twelve fixes that move your inbox rate, from SPF, DKIM and DMARC to list pruning and send time, ordered by how fast each one shows up in your numbers.
5 min readEmail Deliverability
Your Emails Are in Junk. Here's the 4-Step Diagnosis.
Stop guessing. Run these four checks in order. Most junk folder issues trace back to the first one.
4 min readEmail Deliverability
InboxStack Brain: What It Does When Gmail Starts Filtering Your Domain
InboxStack Brain watches your sending domains, opens an incident when Gmail or Outlook starts filtering you, and tells you why it happened and what to fix first. What it does, what it costs, and who it is for.
5 min readCold Email
Cold Email Is Landing in Spam: The Domain, Warmup and Volume Setup That Stops It
Cold email lands in spam when you send from your main domain, skip warmup, or blast a list nobody asked to be on. Dedicated domains, a six-week ramp, daily volume caps and the numbers that mean stop, step by step.
5 min readEmail Deliverability
Why Are My Emails Going to Spam? The Six Causes and the Fix for Each
Your email went out and never reached the inbox. The six reasons Gmail, Outlook and Yahoo file mail as spam, how to tell which one is yours in minutes, and the fix for each, in the order to try them.
6 min readEmail Deliverability
Google Postmaster, Microsoft SNDS and Sender Score: Watching All Three From One Screen
Your Gmail domain score lives in Postmaster, your Outlook data in SNDS, your IP score in Sender Score. InboxStack pulls the three into one view and alerts you the moment any of them moves.
2 min readEmail Deliverability
Google and Yahoo's Bulk Sender Rules: The DMARC Deadline and the 0.3% Spam Line
Send more than 5,000 emails a day to Gmail or Yahoo and you need a DMARC policy and a user-reported spam rate under 0.3%. What the rules require, and how InboxStack helps you stay under the line.
2 min readEmail Deliverability
Setting Up DMARC Without Breaking Your Own Mail: The InboxStack Walkthrough
DMARC tells Gmail what to do with mail that fails SPF or DKIM. Get it wrong and your own mail is what gets rejected. The steps InboxStack walks you through: the record, the DNS entries, and watching the results after.
2 min readEmail Deliverability
InboxStack vs Inbox Monster, Alphabet Email and Postmastery: Which Tool Warns You Before Mail Hits Spam
A short, honest comparison of four tools that watch where your email lands — InboxStack, Inbox Monster, Alphabet Email and Postmastery — and what InboxStack adds: seed tests, alerts and DMARC in one place.
1 min read