Blog
Deliverability, demystified.
Field-tested guides on email authentication, content quality, and reading your WillItInbox report.
Browse by topic
Fake Signups Are Costing You More Than You Think: Prevention with Email
Fake signups drain infra budget, skew metrics, and wreck deliverability. Here's the cost stack and a layered email-validation defense for your SaaS forms.
Read article- ·9 min read
Email List Decay: How Fast Your Database Rots (and the Fix Cadence)
Email list decay runs roughly 20–30% per year — faster for B2B. See the compounding cost math and the exact re-verification cadence that stops the rot.
email list decay rateEmail deliverability - ·9 min read
Catch-All Email Addresses: Accept, Reject, or Segment? A Risk-Based
Should you accept or reject a catch-all email address? Neither blindly. Here's a risk-tier playbook for segmenting and mailing accept-all domains safely.
catch-all email address accept or rejectEmail deliverability - ·10 min read
Email Throttling: Mailbox Provider Rate Limits and How to Pace Sends
Email throttling best practices: why providers return 421/452 deferrals, per-IP and per-domain limits, exponential backoff, and warmup pacing schedules.
email throttling best practicesEmail deliverability - ·9 min read
SMTP vs API for Transactional Email: An Engineer's Honest Comparison
SMTP vs API for transactional email: throughput, retries, idempotency, error granularity, attachments, webhooks — plus when SMTP still wins.
smtp vs api email sendingEmail deliverability - ·10 min read
Dedicated IP vs Shared IP for Email: When to Switch (and When Not To)
Dedicated IP vs shared IP for email: who should switch, the real warmup burden, volume thresholds, cost math, and when dedicated actively hurts you.
dedicated ip vs shared ip emailEmail deliverability - ·10 min read
STARTTLS vs SSL/TLS for Email: Implicit, Opportunistic, and Enforced
STARTTLS vs SSL/TLS for SMTP explained: implicit TLS on 465, STARTTLS upgrade on 587/25, opportunistic vs enforced modes, downgrade attacks, and RFC 8314.
starttls vs ssl tls smtpEmail deliverability - ·10 min read
DKIM Replay Attacks: Why a Valid DKIM Signature Isn't Proof of Anything
DKIM replay attacks explained: how spammers re-inject one legit signed message at scale, why the signature still verifies, and how senders mitigate it.
dkim replay attackEmail deliverability - ·10 min read
Why Corporate Domains Eat Your Emails: Proofpoint, Mimecast, and the SEG
Why emails get blocked by Proofpoint and Mimecast: how secure email gateways inspect, rewrite, and quarantine B2B mail — and how senders get through.
why emails blocked by proofpointEmail deliverability - ·10 min read
Apple Mail Privacy Protection: What It Did to Open Rates (and What to
Apple Mail Privacy Protection inflates open rates with machine opens. Here's what MPP does, which email metrics still work in 2026, and what to measure.
apple mail privacy protection open ratesEmail deliverability - ·10 min read
SMTP Ports 25 vs 465 vs 587: Which One Should You Actually Use?
SMTP port 587 vs 465 vs 25 explained: relay vs submission roles, why ISPs block port 25, RFC 8314 implicit TLS, port 2525 fallback, and a decision table.
smtp port 587 vs 465 vs 25Email deliverability - ·9 min read
DMARC for Subdomains: The sp= Tag, Inheritance Rules, and Common Mistakes
DMARC sp tag for subdomains explained: policy inheritance rules, when a subdomain needs its own _dmarc record, common mistakes, and DNS examples.
dmarc sp tag subdomainEmail deliverability - ·9 min read
List Bombing: The Signup Attack That Trashes Your Sender Reputation
A list bombing attack uses your signup forms to bury victims' inboxes — and destroys your sender reputation. Learn detection signals and layered defenses.
list bombing attackEmail deliverability - ·10 min read
Greylisting Explained: Why Your Emails Arrive Late (and What to Do)
Greylisting is a receiver-side anti-spam technique that delays first-time senders with a 451 temporary failure. Learn how it works and what you can control.
what is greylisting emailEmail deliverability - ·10 min read
Inbox Placement vs Delivery Rate: Why 98% 'Delivered' Lies to You
Email delivery vs deliverability explained: why a 98% delivery rate can hide 30% spam placement, and how to measure inbox placement properly.
email delivery vs deliverabilityEmail deliverability - ·10 min read
How to Read Email Headers: A Complete Analysis Guide
Learn how to read email headers step by step: the Received chain, Authentication-Results, ARC, X- headers, and how to trace an email's true origin IP.
how to read email headersEmail deliverability - ·12 min read
How to Test Transactional Email in Development (Without Emailing Real
Learn how to test transactional emails safely in development using sandbox SMTP, environment isolation, and recipient allowlists — without emailing real users.
test transactional emailsEmail deliverability - ·11 min read
Email Suppression Lists: What They Are and How to Build One That Works
An email suppression list is a permanent do-not-send registry. Learn what belongs on it, how it differs from deletion, and how to build one that survives ESP migrations.
email suppression listEmail deliverability - ·12 min read
Email Typo Detection: Catching 'gmial.com' Before It Becomes a Bounce
Email typo detection catches misspelled domains like gmial.com at signup. Learn how it works, why 1–3% of signups are typos, and how to fix them safely.
email typo detectionEmail deliverability - ·11 min read
Role-Based Email Addresses (info@, support@, sales@): Should You Send to
Role based email addresses like info@ and support@ hurt deliverability more than you think. Learn when to send, when to suppress, and how to detect them.
role based email addressesEmail deliverability - ·10 min read
No-Reply Email Addresses: Why 'noreply@' Quietly Hurts Your Deliverability
A no-reply email address forfeits one of the strongest engagement signals Gmail and Yahoo use. Here's why noreply@ hurts deliverability and what to use instead.
no-reply email addressEmail deliverability - ·11 min read
Single Opt-In vs Double Opt-In: Which Is Actually Better for
Single opt-in vs double opt-in: the real deliverability math, bot-signup risks, GDPR consent evidence, and a decision table to choose right.
single opt-in vs double opt-inEmail deliverability - ·10 min read
Bought an Email List? Here's What Happens Next (and How to Recover)
Bought email list? Here's the math on why purchased lists fail, the damage cascade that follows, and how to recover your sender reputation.
bought email listEmail deliverability - ·10 min read
Gmail Promotions Tab: How to Reach the Primary Inbox (Honestly)
Gmail Promotions tab isn't spam — it's delivered mail. Learn what actually moves email to the primary inbox, what doesn't, and when to stay put.
gmail promotions tabEmail deliverability - ·12 min read
Cold Email Deliverability in 2026: The Technical Playbook
Cold email deliverability in 2026 runs on infrastructure, not copy tricks. Secondary domains, auth, volume discipline, and list quality — the full playbook.
cold email deliverabilityEmail deliverability - ·9 min read
Domain Reputation vs IP Reputation: What Actually Matters in 2026
Domain reputation vs IP reputation in 2026: which one mailbox providers actually filter on, when a dedicated IP is worth it, and how to monitor both.
domain reputation vs ip reputationEmail deliverability - ·11 min read
How to Validate Emails at Signup in Real Time (Stop Bad Addresses at the
Real time email validation at signup: API call patterns, latency budgets, fail-open design, verdict handling matrix, and code engineers can ship today.
real time email validationEmail deliverability - ·9 min read
Email Verification vs Validation: What's the Difference (and Why It
Email verification vs validation: the honest answer, the checks that actually matter, accuracy-claim red flags, and a buyer's checklist.
email verification vs validationEmail deliverability - ·10 min read
Why Your Emails Go to Junk in Outlook and Hotmail (It's Not the Same as
Emails going to junk in Outlook even after Gmail is fixed? Here's why Microsoft filters differently — and the exact SNDS, JMRP, and 5.7.515 fixes.
emails going to junk in outlookEmail deliverability - ·11 min read
WordPress and WooCommerce Emails Going to Spam? Here's the Real Fix
WordPress emails going to spam? The root cause is wp_mail() and your host's server. Here's the 5-step fix that actually works, with exact DNS records.
wordpress emails going to spamEmail deliverability - ·11 min read
Why Did My Email Open Rates Suddenly Drop? A Diagnostic Playbook
Email open rates dropped overnight? Run this structured diagnostic: rule out tracking artifacts first, then isolate Gmail, Outlook, or sending-side causes.
email open rates droppedEmail deliverability - ·9 min read
SPF 'Too Many DNS Lookups' (PermError): What It Means and How to Fix It
SPF too many DNS lookups error explained: why the 10-lookup limit exists, which mechanisms count, and 4 ranked fixes with dig examples.
spf too many dns lookupsEmail deliverability - ·10 min read
Does Email Warmup Actually Work? An Honest Look at Warmup Services
Does email warmup work? The honest answer: IP/domain ramping is real, but automated engagement networks are shaky. Here's what actually works.
does email warmup workEmail deliverability - ·10 min read
How to Read a DMARC Aggregate Report (rua XML) — A Practical Guide
Learn how to read DMARC reports (rua XML) in minutes. Decode report metadata, records, and auth results — and spot the 4 patterns that matter.
how to read dmarc reportEmail deliverability - ·12 min read
Why Your Signup and Password-Reset Emails Land in Spam (and How to Fix It)
Signup emails going to spam? Learn how to diagnose whether it's a sending, delivery, or placement problem — and the exact fix sequence that works.
signup emails going to spamEmail deliverability - ·10 min read
SMTP Error Codes Explained: 550, 551, 552, 554 (and What to Do About Each)
SMTP error codes decoded: what 550, 551, 552, and 554 bounces mean, when to retry, and when to suppress the address permanently.
smtp error codesEmail deliverability - ·10 min read
Gmail '550 5.7.26' Error: What It Means and How to Fix It
Getting Gmail's 550 5.7.26 error? Learn what each variant means, why Gmail rejects unauthenticated mail, and the exact DNS fixes that resolve it.
550 5.7.26Email deliverability - ·11 min read
What Are Spam Traps? Types, Detection, and How to Avoid Them
What are spam traps? Learn the 3 types (pristine, recycled, typo), how they get into your list, and the defenses that actually work.
what are spam trapsEmail deliverability - ·10 min read
Disposable Email Addresses: How to Detect and Block Them (Without Blocking
Disposable email address detection for SaaS: how temp email services fuel fake signups and trial abuse, and how to block them without rejecting real users.
disposable email address detectionEmail deliverability - ·11 min read
How to Verify an Email Address Without Sending an Email
Learn how to verify an email address without sending email: syntax, DNS, MX, SMTP handshake checks, catch-all detection, and spam-trap signals — explained layer by layer.
verify email address without sending emailEmail deliverability - ·2 min read
Deliverability QA in CI/CD with the WillItInbox API
How developers can turn deliverability testing and validation into repeatable checks around releases, templates, and imports.
APICI/CDDevelopers - ·12 min read
Inbox placement testing: seed tests and repeat checks
Use seed inboxes to collect diagnostic placement samples, compare repeat tests, and avoid treating one run as a universal inbox guarantee.
Inbox placementSeed testsDeliverability - ·2 min read
DMARC aggregate reports: find unknown senders
Use RUA report evidence to label known senders, investigate unknown sources, and avoid breaking legitimate mail during policy rollout.
DMARCMonitoringAuthentication - ·2 min read
Bulk email validation CSV checklist
A practical checklist for uploading, validating, segmenting, and exporting CSV email lists without damaging sender reputation.
Bulk validationCSVList hygiene - ·3 min read
Email validation API status guide
Email validation API results explained: act on valid, invalid, risky, catch-all, and unknown statuses using 12-layer evidence and confidence scores.
APIValidationList hygiene - ·3 min read
How to check if an email will go to spam before launch
Test the final production-like message, separate durable failures from warnings, and retest the same sender path before launch.
DeliverabilityLaunch checklistSpam test - ·5 min read
Gmail & Yahoo Bulk Sender Rules 2026: The Complete Compliance Checklist
Check the Gmail and Yahoo bulk sender rules for SPF, DKIM, DMARC, one-click unsubscribe, complaint rate, TLS, PTR, and list quality before scaling.
DeliverabilityGmailYahooCompliance - ·13 min read
DKIM key rotation: a safe, zero-downtime playbook
Why rotating DKIM keys matters, when to do it, and the exact two-selector cutover that keeps signatures valid through the change.
DKIMAuthenticationDNS - ·5 min read
SPF record syntax: mechanisms, examples, and lookup limits
Use practical SPF examples to understand mechanisms, qualifiers, includes, redirects, DNS lookup limits, and common failure modes.
SPFAuthenticationDNS - ·7 min read
DMARC rollout: from p=none to p=reject without breaking mail
An evidence-led path from monitoring-only DMARC to tested enforcement under RFC 9989, with checkpoints that protect legitimate mail.
DMARCAuthentication - ·4 min read
SPF, DKIM, and DMARC explained for sender authentication
Understand how SPF, DKIM, and DMARC work together, why alignment matters, and when to move from a checker to monitoring.
AuthenticationDNS - ·14 min read
BIMI email requirements: DMARC, VMC, CMC, and setup
Review BIMI prerequisites, logo requirements, certificate options, DNS publication, and the limits of mailbox-provider display.
BIMIAuthenticationBranding - ·14 min read
MTA-STS and TLS-RPT: setup, testing, and failure diagnosis
Publish MTA-STS safely, collect TLS reports, diagnose transport failures, and move from testing to enforcement with evidence.
TLSMTA-STSAuthentication - ·2 min read
DMARC alignment explained: SPF, DKIM, and From domains
DMARC alignment explained: why DMARC passes when SPF fails, how relaxed vs strict alignment works, and how to read alignment errors in aggregate reports.
AuthenticationDMARC - ·4 min read
ARC email authentication: how forwarding preserves results
Understand ARC sets, chain validation, forwarding scenarios, and why ARC preserves authentication evidence without making DMARC pass.
ARCDMARCAuthentication - ·4 min read
DNSSEC for email: where it helps and what it cannot fix
Learn how DNSSEC protects DNS answers, where it supports email security, and why it does not directly guarantee authentication or inbox placement.
DNSSECDNSAuthentication - ·3 min read
Why Emails Land in Spam: 6 Causes and How to Fix Each
Diagnose spam-folder problems by separating sender reputation, list quality, authentication, content, links, and provider-specific evidence.
DeliverabilityContent - ·15 min read
Bounces decoded: hard, soft, block, and how to handle each
What every SMTP bounce code actually means, which ones to retry, and the suppression rules that keep your reputation intact.
BouncesDeliverabilitySMTP - ·13 min read
Feedback loops (FBLs): track and suppress complaints
Every major receiver's FBL program, how to enroll, and the suppression discipline that turns complaints into long-term placement gains.
FBLDeliverabilityReputation - ·4 min read
IP warming schedule: ramp volume with provider evidence
Plan gradual sending increases with reputation, complaint, authentication, placement, and list-quality evidence instead of fixed-volume folklore.
IP WarmingReputationDeliverability - ·13 min read
Microsoft SNDS monitor guide for Outlook sender reputation
Use Microsoft SNDS to interpret Outlook complaint, trap-hit, filter, HELO, and IP reputation evidence before sender issues become blocks.
SNDSMicrosoftOutlookReputation - ·3 min read
Image-to-Text Ratio in Emails: Does It Affect Spam Filters? (Tested)
There is no universal image-to-text cutoff. Learn what image-heavy email can break, which signals matter, and how to test the final MIME message.
HTML EmailSpam FiltersDesign - ·14 min read
PTR records and reverse DNS for email: setup and checks
Configure forward-confirmed reverse DNS, align HELO with the PTR hostname, and diagnose the infrastructure evidence receivers inspect.
PTRrDNSInfrastructure - ·13 min read
List-Unsubscribe Header: One-Click Unsubscribe Setup (RFC 8058)
Add List-Unsubscribe and List-Unsubscribe-Post headers, build the RFC 8058 POST endpoint, avoid common Gmail/Yahoo mistakes, and test compliance.
List-UnsubscribeRFC 8058GmailYahoo - ·14 min read
Google Postmaster Tools guide: setup, metrics, and fixes
Set up Google Postmaster Tools, interpret reputation and compliance data, and connect each dashboard signal to a defensible next step.
Postmaster ToolsGmailReputation - ·11 min read
Email subdomain strategy for safer sending
Splitting transactional, marketing, and corporate mail across subdomains protects your most important traffic from your noisiest. Here is the layout that works.
SubdomainsReputationArchitecture - ·5 min read
Blacklist removal playbook for email senders
Which DNSBLs actually matter, how to find your listings, and the exact delisting process for the eight most consequential blocklists.
DNSBLBlacklistReputation - ·3 min read
MX Records Explained: Setup, Priority, and Common Mistakes
Learn what MX records do, how priority and fallback work, which DNS mistakes break inbound mail, and how to verify MX routing safely.
MXDNSMail Routing - ·2 min read
Accessible HTML email: structure, contrast, and testing
Create accessible email with semantic structure, readable contrast, useful alternative text, and production-like client testing.
AccessibilityHTML EmailDesign - ·3 min read
Plain text vs HTML email: why both matter
Why HTML-only emails get filtered, what a clean multipart/alternative looks like, and the four-line MIME structure to copy.
ContentMIMEDeliverability - ·3 min read
How to read a WillItInbox deliverability report
Prioritize authentication, routing, headers, content, links, and infrastructure findings without treating the score as a delivery guarantee.
WillItInboxDeliverability reportSpam test - ·14 min read
Email spam trigger words: what SpamAssassin actually scores
See why isolated words rarely decide placement, which structural and phrase combinations score, and how to test the final message.
ContentSpamAssassinCopywriting - ·4 min read
Email list hygiene: suppress, sunset, or remove
Build a list-hygiene policy for invalid, inactive, risky, and unengaged recipients without confusing validation with consent.
List HygieneEngagementDeliverability - ·2 min read
Transactional vs marketing email — separate them or suffer
Transactional vs marketing email: why mixing both streams on one domain is a deliverability liability, and how to split subdomains, IPs, and tracking cleanly.
OperationsReputation