Resources
Deliverability, SMTP, security and developer guides, from the people running the infrastructure.
Gmail, Yahoo and Microsoft sender rules: the 2026 state of play
Authentication, alignment, complaint rates and one-click unsubscribe - what the big four now require, and why non-compliant mail is rejected outright rather than filtered.
Gmail & Yahoo sender rules: what bulk senders must do
Authenticate with SPF and DKIM, publish DMARC, keep complaints under 0.3%, and support one-click unsubscribe.
DeliverabilityDKIM passes but mail still lands in spam — why
Authentication is necessary, not sufficient. Reputation and engagement decide the folder.
DNSThe SPF 10-lookup limit and how to stay under it
Too many include: mechanisms cause a permerror — and a permerror is an SPF failure.
DNSDMARC explained: none, quarantine, reject
DMARC ties SPF and DKIM to your From domain and tells receivers what to do on failure.
DeliverabilityHard vs soft bounces, and why you must suppress hard ones
A hard bounce is permanent. Keep mailing it and you look like a spammer.
ProductMessage streams: keep transactional and broadcast apart
Mixing a marketing blast with password resets lets one bad campaign sink your critical mail.
APIWebhooks done right: verify signatures and deduplicate
Every event is signed and may be retried — verify the HMAC and dedupe on the event id.
DeliverabilityWarming up a new sending IP without getting blocked
Ramp volume gradually so receivers build trust in your pattern.
SMTPWhich SMTP port should you use: 587, 465 or 2525?
587 with STARTTLS is the modern default; 465 is implicit TLS; 2525 is a fallback when others are blocked.
DeliverabilityFive ways to cut your complaint rate below 0.1%
Only mail people who asked, make unsubscribe trivial, and honour it instantly.
Email VerificationHow to Prevent Fake Signups and Bot Registrations
How to prevent fake signups and bot registrations with layered defenses: email checks, rate limits, invisible challenges, honeypots, and confirmed access.
Developer GuidesHow to Do an MX Lookup in Node.js (dns.promises.resolveMx)
MX lookup in Node.js with dns.promises.resolveMx: sorting, error codes, null MX, implicit MX fallback, timeouts, caching, and Unicode domains, with tested code.
DeliverabilityGmail's Spam Rate Threshold: Staying Below 0.3% (and Aiming for 0.1%)
Gmail spam rate explained: how Google calculates it, why 0.3% is the limit and 0.1% the target, what happens if you cross it, and how to lower it.
List HygieneEmail Sunset Policy: When to Stop Emailing Inactive Subscribers
Build an email sunset policy: define inactivity, set stages for reduced sending and re-engagement, choose exceptions, and automate removing unengaged contacts.
Cold EmailGDPR and B2B Cold Email: What You Need to Know
GDPR cold email rules for B2B prospecting: legitimate interest, what your first email must disclose, the right to object, and key country differences.
DeliverabilityDMARC Alignment: Strict vs Relaxed Explained
DMARC alignment decides whether SPF and DKIM results count for your From domain. Learn strict vs relaxed modes, organizational domains, and which to use.
DeliverabilityEmail Blacklists (Blocklists): How to Check Them and Get Delisted
Email blacklist check guide: how DNS blocklists work, which lists affect delivery, how to check your IP and domain, and what to fix before delisting.
Deliverability"550 5.1.1 User Unknown": What It Means and How to Fix It
550 5.1.1 user unknown means the mailbox you emailed doesn't exist. Learn what causes it at Gmail, Outlook and Yahoo, and how to fix it as a sender or admin.
Email VerificationEmail Verification Results Explained: Valid, Invalid, Risky, Unknown
Email verification results explained: what valid, invalid, risky, catch-all, and unknown mean, which SMTP codes sit behind them, and what to do with each one.
Email VerificationHow to Block Disposable Emails at Signup (Without Losing Real Users)
How to block disposable emails at signup: domain lists, server-side checks, clear error messages, and proportionate rules that keep real users signing up.
Cold EmailCAN-SPAM and Cold Email: What US Law Requires
CAN-SPAM cold email rules for US B2B outreach: what each prospecting email must include, how to handle opt-outs in sequences, and the penalties.
Email VerificationCatch-All (Accept-All) Email Domains: What They Are and How to Handle Them
A catch-all email domain accepts mail for any address, so no verifier can confirm the mailbox. Learn how catch-all works and how to send to these contacts.
Cold EmailCold Email Deliverability: How to Stay Out of Spam
Cold email deliverability depends on authentication, clean lists, modest volume, and low complaints. A practical guide to reaching the inbox the right way.
Cold EmailCold Email Sending Limits per Day: Safe Volumes
Cold email sending limits per day for Gmail, Google Workspace, and Microsoft 365, plus safe volumes per mailbox and how to ramp up a new domain without spikes.
DeliverabilityDKIM Explained: How Email Signing Works
DKIM adds a cryptographic signature to every email so receivers can verify the sender. See how DKIM signing works, the DNS key record, and common failures.
DeliverabilityDMARC Explained: Policies, Tags, Reports, and Rollout Plan
DMARC tells receivers what to do when mail fails SPF and DKIM alignment. Learn the policies, every record tag, how reports work, and what RFC 9989 changed.
DeliverabilityDomain Reputation vs IP Reputation: What Matters More for Email
Domain reputation vs IP reputation: how each is built, which one mailbox providers weigh more today, how to check both, and how to fix the weak one.
List HygieneDouble Opt-In vs Single Opt-In: Which Should You Use?
Double opt-in vs single opt-in compared: how each works, what Gmail, Yahoo, AWS and M3AAWG recommend, the effect on list quality, and when single opt-in works.
List HygieneEmail List Decay: Why Email Lists Go Bad Over Time
Email list decay explained: why addresses stop working, how job changes and abandoned inboxes drive it, and how to measure your own decay rate in a spreadsheet.
List HygieneEmail List Hygiene: A Complete Guide for Marketers and Founders
Email list hygiene explained: keep bad addresses out, remove bounces and inactive contacts, and set a cleaning schedule that protects your sender reputation.
Developer GuidesEmail Validation Regex: A Practical Pattern and Why Regex Alone Isn't Enough
A tested email validation regex for JavaScript, Python, and PHP, what it catches, what it misses, and why a pattern can never prove an address receives mail.
Email VerificationEmail Validation vs Email Verification: What's the Difference?
Email validation vs verification: validation checks that an address is well formed, verification checks it can receive mail. See when you need each one.
Email VerificationHow Accurate Is Email Verification? What the Results Really Mean
Email verification accuracy depends on which check produced the result. Learn which results are certain, which are guesses, and how to test a verifier yourself.
Developer GuidesEmail Verification API: How to Integrate It into Signups and List Cleaning
How to integrate an email verification API: real-time signup checks, bulk list cleaning, timeouts, rate limits, and acting on valid, risky, and invalid results.
Email VerificationEmail Verification for Ecommerce Stores: Checkout, Receipts, and Marketing Lists
Email verification for ecommerce: catch typos at checkout, keep order confirmations deliverable, and keep your marketing list clean without adding friction.
Email VerificationEmail Verification for Nonprofits and Fundraising Lists
Email verification for nonprofits: clean donor, volunteer, and petition lists before year-end appeals so donation receipts and campaigns reach supporters.
Email VerificationEmail Verification for Real Estate Agents: Cleaning Lead Lists and CRM Imports
Email verification for real estate: clean open house sign-ins, portal leads, and CRM imports so your drip campaigns reach buyers and sellers, not bounces.
Email VerificationEmail Verification for Recruiters and Staffing Agencies
Email verification for recruiters: clean sourced candidate lists, catch dead work emails, and protect your sending domain before outreach to talent and clients.
Email VerificationEmail Verification for SaaS Signups and Onboarding
Email verification for SaaS: stop fake trial signups, catch typos before onboarding emails bounce, and score leads by domain without blocking real users.
DeliverabilityEmail Warm-Up: How to Warm Up a New Domain or IP Safely
Email warm up guide for a new domain or IP: how long it takes, a sample ramp schedule, who to email first, and the bounce signals that mean slow down.
DeliverabilityGoogle Postmaster Tools: A Practical Guide for 2026
A practical Google Postmaster Tools guide: set up and verify your domain, read the spam rate and compliance dashboards, and act on what the data shows.
DeliverabilityGmail and Yahoo Bulk Sender Requirements Explained (With a Self-Audit)
Gmail and Yahoo sender requirements explained line by line, with header and DNS examples and a step-by-step self-audit you can run before your next campaign.
DeliverabilityHard Bounce vs Soft Bounce: Causes and Fixes
Hard bounce vs soft bounce: what causes each, the SMTP codes behind them, the gray areas that fool most senders, and the right fix for every case.
Email VerificationHow to Check If an Email Address Is Valid (7 Methods)
Seven ways to check if an email is valid, from spotting format errors and typos to MX lookups, disposable domain checks, SMTP tests, and confirmation links.
List HygieneHow to Clean an Email List in 6 Steps
How to clean an email list in 6 steps: export, fix formatting, remove duplicates, verify addresses, suppress bounces, and handle inactive subscribers safely.
DeliverabilityHow to Reduce Your Email Bounce Rate: A Practical Checklist
A practical checklist to reduce email bounce rate: validate at signup, clean old lists, authenticate, throttle by provider, and handle every bounce correctly.
Email VerificationHow to Verify an Email Address Without Sending an Email
Verify an email address without sending a message: check syntax, look up MX records with dig or nslookup, run a manual SMTP test, and know when results lie.
DeliverabilityOne-Click Unsubscribe (RFC 8058) and the List-Unsubscribe Header
How one-click unsubscribe works under RFC 8058: the List-Unsubscribe headers, DKIM signing, a working endpoint example, and testing at Gmail and Yahoo.
DeliverabilityOutlook Deliverability: Getting Into the Outlook and Hotmail Inbox
Outlook deliverability guide for Outlook.com and Hotmail: Microsoft's sender rules, the technical checks, SNDS and JMRP, and fixes when mail lands in Junk.
List HygieneRe-Engagement Email Campaigns: Win Back or Remove Inactive Subscribers
How to run a re-engagement email campaign: define inactive subscribers, verify the segment, write a 3-email win-back sequence, and remove non-responders safely.
Email VerificationRole-Based Email Addresses (info@, sales@): Should You Email Them?
Role-based email addresses like info@ and sales@ reach a team, not a person. Learn when it is fine to email them, when to avoid them, and how to flag them.
Cold EmailUsing Secondary Domains for Cold Email: Setup and Honest Use
How to use a secondary domain for cold email: when it makes sense, how to pick and authenticate it, and how to keep it transparent and compliant with US law.
DeliverabilitySender Reputation: What It Is and How to Protect It
Sender reputation explained: what Gmail, Yahoo, and Microsoft track about your domain and IP, the signals that damage it, and a practical routine to protect it.
DeliverabilitySMTP Bounce Codes Explained: 4xx and 5xx Reply Codes
SMTP bounce codes explained: how 4xx and 5xx reply codes and enhanced status codes work, plus a reference of real Gmail, Microsoft, and Yahoo errors.
Email VerificationHow to Avoid Spam Traps: List Practices That Keep Them Off Your List
How to avoid spam traps with better list practices: confirmed opt-in, typo checks, sunsetting inactive contacts, bounce handling, and never buying lists.
DeliverabilityHow to Set Up SPF, DKIM, and DMARC Step by Step
A step-by-step SPF DKIM DMARC setup guide with exact DNS records for Google Workspace, Microsoft 365, and Amazon SES, plus how to test each one before you send.
DeliverabilitySPF Record Explained: Syntax, Examples, and Common Mistakes
An SPF record lists the servers allowed to send mail for your domain. Learn the syntax, see real examples, and fix the mistakes that cause SPF failures.
DeliverabilitySPF "Too Many DNS Lookups" (PermError): How to Fix It
Getting SPF too many DNS lookups or a PermError? Learn how the 10-lookup limit is counted, how to audit nested includes, and the safe ways to get back under it.
Developer GuidesHow to Validate an Email Address in JavaScript (Browser and Node.js)
Validate email in JavaScript: the browser's built-in check, a tested regex, validator.js, and a Node.js MX lookup that confirms the domain accepts mail.
Developer GuidesHow to Validate an Email Address in PHP (filter_var, DNS, and Beyond)
Validate email in PHP 8.3 with filter_var, add an MX check with getmxrr, and handle Unicode, IP literals, and null MX, with tested code and honest limits.
Developer GuidesHow to Validate an Email Address in Python (email-validator, Regex, and DNS)
Validate email in Python with the email-validator package, a regex, and a dnspython MX lookup, with runnable code and the limits each method can't get past.
Cold EmailWhy You Should Verify Emails Before Cold Outreach
Why you should verify emails before cold email campaigns: bounces damage sender reputation, trigger provider limits, and waste sends. What to check and how.
Email VerificationWhat Is a Disposable Email Address? Types, Uses, and Risks
A disposable email address is a short-lived or throwaway inbox. Learn the main types, why people use them, and how businesses should handle them fairly.
DeliverabilityWhat Is a Good Email Bounce Rate?
What is a good email bounce rate? See the published limits from Amazon SES, HubSpot and Postmark, how to calculate your rate correctly, and when to worry.
DeliverabilityWhat Is an Email Bounce? Hard vs Soft Bounces Explained
An email bounce is a message the receiving server refused or could not deliver. Learn how bounces work, how to read one, and hard vs soft bounces.
DeliverabilityWhat Is Email Deliverability? A Beginner's Guide for Senders
Email deliverability explained for beginners: delivered vs. inboxed, what Gmail, Yahoo, and Microsoft judge, and the first fixes that improve results.
Email VerificationWhat Is Email Verification and How Does It Work?
Email verification checks whether an address is real and safe to send to. Learn each check, from syntax and MX records to SMTP, and what it can't tell you.
DeliverabilityWhy Are My Emails Going to Spam? 12 Common Causes and Fixes
Why are my emails going to spam? Twelve common causes, from failed SPF, DKIM, and DMARC to bad lists and complaints, with how to diagnose each one and fix it.
DeliverabilityYahoo and AOL Deliverability Tips for 2026
Yahoo deliverability tips for Yahoo and AOL inboxes: sender requirements, Sender Hub, the Complaint Feedback Loop, TS deferrals, and list practices.
SMTPSTARTTLS vs implicit TLS: which port should you use?
A practical guide to 587, 465 and 2525, and why legacy servers sometimes need cleartext auth.
SecurityLocking down API keys with IP allowlists and scopes
Least-privilege keys, per-domain restrictions and rotating credentials without downtime.
Developer GuidesSending your first email with the REST API
From API key to a delivered, DKIM-signed message in under five minutes.
DeliverabilityBounce classification: hard, soft, and what to do about each
How auto-suppression and retries protect your reputation, and when to intervene manually.
Product UpdatesTemplates and Message Streams are here
Reusable copy with variables, and a clean split between transactional and broadcast mail.
Developer GuidesHandling inbound email with routes and webhooks
Forward, store or POST incoming mail to your app, matched by recipient, sender or subject.
DeliverabilityPostmaster Tools v2 retired reputation scores. Now what?
Google removed the domain and IP reputation dashboards. Spam rate and compliance status are what is left, and they are better signals than the colour-coded score ever was.
DeliverabilityMicrosoft now rejects non-compliant bulk mail outright
Outlook, Hotmail and Live moved from filtering non-compliant bulk mail to refusing it. What the requirements are, how they differ from Gmail, and what a rejection looks like in your logs.
SecurityMTA-STS and TLS-RPT: 0.7% adoption, half your traffic
Only a fraction of domains publish an MTA-STS policy, yet it can cover most of your outbound mail because the large providers implemented it first. How to deploy it in testing mode and what the reports tell you.
SMTPWhy EC2 blocks port 25, and what actually works
AWS allows outbound port 25 to private addresses only. The restriction, the removal request, and the three practical routes out when the request is refused.
DeliverabilitySPF flattening solves the lookup limit and creates a worse one
Flattening replaces includes with the IP addresses behind them. It gets you under ten lookups and quietly breaks the next time a provider changes infrastructure.
SecurityRotating a DKIM key without breaking mail in flight
The safe order is publish, wait, switch, wait, remove. Doing it in any other order invalidates signatures on messages that are still being delivered.
DeliverabilityAlignment is the DMARC rule that fails while everything passes
SPF pass plus DKIM pass does not mean DMARC pass. The authenticated domain has to match the one the recipient sees, and that is where shared sending infrastructure breaks.
DeliverabilityGetting from p=none to p=reject without losing mail
A staged DMARC rollout: what to look for in the reports at each step, when to move the policy, and why the subdomain policy should go first.
DeliverabilityHow to read a DMARC aggregate report
What each field in the XML means, which ones matter, and how to tell a forwarding artefact apart from a genuine authentication failure.
SecurityARC, and why forwarded mail fails authentication
Forwarding breaks SPF and mailing lists break DKIM. ARC records that a message authenticated before it was relayed, and adoption determines whether that helps.
DeliverabilityBIMI and VMC: what it costs and what you actually get
A logo beside your messages, after DMARC enforcement, a specific SVG profile and an annual certificate. Whether that chain is worth completing depends on who your recipients are.
DeliverabilityWhy you should not send from your root domain
Subdomains separate reputation, isolate failures and give each stream its own SPF budget. How to split them and what each one should carry.
SMTPVERP: knowing which message bounced
A bounce arrives as a new message with no reliable link to what caused it. Encoding the recipient in the return-path makes the link unambiguous.
DeliverabilityWhat belongs on a suppression list, and for how long
Complaints are permanent. Hard bounces are permanent. Everything else is a judgement call, and getting it wrong costs you either reputation or recipients.
DeliverabilityFeedback loops: getting complaints back, and acting on them
Most providers will tell you when a recipient marks your mail as spam, if you register. What each one offers, and what to do with the reports.
DeliverabilityDedicated or shared IPs: the volume question
A dedicated IP gives you a reputation of your own, which is only an advantage once you send enough to build one. Below that threshold it is strictly worse.
SMTPRate limits are per provider, and ignoring that costs you
Every major receiver limits connections and messages per unit time. Backing off when told to is cheaper than being deferred, and far cheaper than being blocked.
DeliverabilityListed on a blocklist: what to do, in order
Find out which list, find out why, fix the cause, then request removal. Requesting removal first is how senders end up listed again a week later.
SMTPRetry schedules: how long to keep trying
A 4xx means try again. How often, for how long, and why transactional mail should give up earlier than the defaults suggest.
Developer GuidesLogging email so you can answer "where did it go?"
The answer a customer needs is the remote server response, verbatim, with a timestamp. Most systems log everything except that.
Developer GuidesIdempotency keys, and why your retry sent two emails
A timeout is not a failure. Without an idempotency key, the safe retry you wrote is the reason a customer received the same receipt twice.
Developer GuidesVerifying webhook signatures, and the mistakes that void them
Your webhook endpoint is a public URL that mutates state. Signature verification is what stops anyone from posting to it, and three common implementation errors make it decorative.
Developer GuidesA batch returned 200. Three of the messages failed.
HTTP status describes the request, not the work. Batch endpoints report per-message outcomes in the body, and code that checks only the status silently drops mail.
Developer GuidesSMTP or REST API: which to integrate against
SMTP is a drop-in replacement for code you already have. The API gives you message IDs, structured errors and batching. The deciding factor is usually what you need back.
Developer GuidesTesting transactional email without sending any
A test suite that sends real mail is slow, flaky and occasionally embarrassing. What to assert at each layer, and where a real send is genuinely required.
DeliverabilityOpen rates after Mail Privacy Protection
Apple pre-fetches images for a large share of recipients, so opens are recorded whether or not anyone looked. What the number still tells you, and what to measure instead.
DeliverabilityWhat makes transactional mail get filtered
Transactional mail is the most wanted mail you send and it still lands in spam. Usually for reasons that have nothing to do with the message.
SMTPWhat self-hosting a mail server actually involves
The software is free and the installation is an afternoon. The IP reputation, the port 25 policy and the ongoing operations are the parts that decide whether it works.
DeliverabilitySpam traps: the two kinds and how they got on your list
A pristine trap means you acquired addresses you were not given. A recycled trap means you kept one too long. The fix is different for each.
Developer GuidesReceiving email as application input
Inbound routing turns an address into a webhook. The hard parts are MIME, forged senders and the fact that anyone can post to your address.
Developer GuidesSending on behalf of your customers, correctly
Multi-tenant sending means per-customer domains, per-customer DKIM keys and per-customer suppression. Sharing any of those shares the consequences too.
DeliverabilitySending from a new domain: the checklist
Everything to publish and verify before the first real message leaves, in the order that avoids waiting for DNS twice.
Developer GuidesTransactional email that works with a screen reader
Semantic structure, real text, meaningful link text and sufficient contrast. Most of it is the same work that keeps you out of the spam folder.
SecurityConsent, records and the rules that outlive your list
GDPR, CAN-SPAM and PECR ask different questions. What they agree on is that you should be able to show where an address came from.