Short technical guides. Every claim here is something you can verify yourself with a DNS lookup.
The free DMARC checker looks up your SPF, DKIM, DMARC and MX records in seconds. No signup. Just checking SPF? The SPF checker shows the exact record and flags the 10-lookup and -all issues. Just checking DKIM? The DKIM checker finds your key and flags weak or missing keys. Worried your IP is blacklisted? The IP blocklist checker tests it against 9 major lists. Not sure if your domain even receives mail? The MX record checker looks up your MX records, resolves each mail server to its IP and checks reverse DNS. Got a suspicious email and want to know if it's real? The email header analyzer reads the raw headers and tells you whether SPF, DKIM and DMARC passed. Getting a weekly DMARC XML you cannot make sense of? The DMARC report parser turns it into a who's-sending-as-you list. Need the record itself? The DMARC record generator writes the exact TXT for you.
Open the DMARC checkerEmails not arriving? The six causes in the order they actually happen: broken SPF, DKIM or DMARC, provider rejections, blocklists, no domain reputation, and list rot. How to find yours in minutes.
The complete setup for sales and outbound email deliverability: dedicated sending domains, SPF, DKIM and DMARC, warm-up, volume rules and monitoring, written for agencies and sales teams.
The best email deliverability tools for 2026, compared by use case: free domain audits, monitoring, DMARC, and cold email.
What to include in a client email deliverability report: the checks, the score, the fixes in priority order, and how to present it so clients act.
How to build email deliverability monitoring: which records to poll, how often to check, what to alert on, and how to act when a check flips.
How healthcare organizations keep patient, provider and billing email out of spam: SPF, DKIM, DMARC, volume and list hygiene.
How real estate teams keep listing alerts, lead follow-up and transaction email in the inbox: SPF, DKIM, DMARC, and list hygiene.
How schools and universities keep student, faculty and alumni email out of spam: SPF, DKIM, DMARC, volume and the new-student problem.
Four free, no-auth REST endpoints that run the same SPF, DKIM, DMARC, MX, TLS and blocklist checks as the web audit. curl examples, response shapes and rate limits.
BIMI puts your logo next to your email in the inbox. Here is what it actually takes, the DMARC prerequisite that blocks most brands, and the exact record to add.
B2B email deliverability is different from B2C. Business inboxes have stricter filters, higher bounce rates, and different engagement patterns. What to check and what to fix.
Email compliance is not optional. What each major regulation requires, what happens when you get it wrong, and how to stay compliant.
A dirty list is the fastest way to tank your deliverability. What to remove, how often to clean it, and the tools that do it for you.
SendGrid is the full platform with a shared IP pool. Postmark is transactional-only with a reputation-first model. What each costs, what each buys, and which one fits.
Mailgun gives you a flexible, cheap shared-IP workhorse. Postmark gives you a reputation-first transactional service. What the difference costs and buys.
Both are shared-IP workhorses. The difference is marketing features, pricing and ecosystem. How to choose between them.
Resend is the new developer-first transactional API. Postmark is the reputation-first veteran. Both are transactional-only, but they optimize for different things.
SendGrid is the mature full-stack email platform. Resend is the lean developer-first transactional API. How the two differ and which fits your product.
Mailgun is the flexible, cheap shared-IP workhorse. Resend is the developer-first transactional API. How they differ and which one fits.
The real SendGrid alternatives, who should actually switch, and the migration checklist that keeps your domain out of spam during the move.
Postmark users usually stay for deliverability. When switching makes sense, what the alternatives are, and the migration checklist that keeps your domain clean.
Mailgun is cheap and flexible, but shared-IP reputation and the lack of marketing focus are real limits. The alternatives and the safe migration path.
Resend is the cleanest developer experience in transactional email, but it is young and narrow. The alternatives and when each one makes sense.
SendGrid shared IPs, its terms, and why cold outreach from the platform lands in spam. What actually works for cold email, and how to set it up.
Postmark is transactional-only by design and its terms prohibit unsolicited bulk mail. Why cold email on Postmark is a bad idea, and what to use instead.
Mailgun is more permissive than Postmark but its shared IPs and terms still make cold email risky. The minimum viable setup if you do it anyway.
Resend is transactional-only by design, its terms restrict unsolicited mail, and its shared IPs are new. Why cold email on Resend fails, and what to use instead.
Mail-Tester, GlockApps, Litmus, MXToolbox, Sender Score and Inboxproof compared: what each checks, what it costs, and which to use for cold email.
SendGrid is accepting your mail but recipients are not seeing it. The five reasons SendGrid deliverability drops, and the exact fix for each one.
Postmark accepts your mail but recipients never see it. The five reasons Postmark deliverability drops, and the exact fix for each one.
Mailgun accepts your mail but recipients never see it. The five reasons Mailgun deliverability drops, and the exact fix for each one.
Resend accepts your mail but recipients never see it. The five reasons Resend deliverability drops, and the exact fix for each one.
The best free and paid alternatives to Mail-Tester, by use case: full domain checks, single-purpose lookups, and inbox placement.
Compare GlockApps with InboxMonster, 250ok, Litmus and the free checkers, and see when you actually need paid inbox-placement testing.
Why order confirmations and marketing emails from your store land in spam, and the exact DNS and reputation fixes for Shopify, WooCommerce and headless.
The DNS records to look up, what each result means, and the order to check them in. No test-email guessing.
The five areas that decide inbox placement: DNS records, sending infrastructure, list hygiene, content, and monitoring. Run it before every launch.
The checks that matter, what is worth paying for, and how to tell a real tool from a marketing page.
The order that matters: fix authentication, clear blocklists, correct the sending behavior, then monitor so it never comes back.
Reply rate is decided before your first word of copy: by whether the email reaches the primary inbox. Here is the order to fix it.
What each Postmaster Tools metric means, how to read reputation, and what to do when a number moves the wrong way.
A practical comparison of Instantly, Smartlead, Lemlist, Woodpecker, Reply.io and Apollo, and the sending-domain checks that decide whether any of them land in the inbox.
The deliverability stack for agencies: per-domain authentication, daily monitoring of every client domain, and white-label reports you can hand over.
Donor appeals, event invites and volunteer email land in spam when your domain is not authenticated. The five fixes, in order, on a small budget.
The deliverability stack for MSPs: per-domain authentication, daily monitoring of every client domain, and client-ready reports that turn a background task into a visible service.
What to monitor, how often, and what to do when an alert fires. The daily loop that keeps every sending domain inbox-ready.
The seven checks that decide delivery, why a score without the records is a black box, and how to tell a real audit from a marketing page.
Sent is not delivered. The records you own even when you send through a provider, why "it worked yesterday" is not a guarantee, and how to monitor so a broken record never kills your user experience.
Keep password resets, receipts and alerts out of spam: dedicated warmed-up IP, SPF/DKIM/DMARC setup, near-zero bounce rates, and daily monitoring.
The setup that keeps cold outreach in the primary inbox: a dedicated sending domain, correct authentication, slow warmup, clean lists, and daily monitoring.
Where the authentication results live in raw headers, what each value means, and the one-minute test to tell a real email from a spoof.
What the record looks like, where it goes in your DNS, which policy to start with, and how to confirm it is live. With a free generator that writes it for you.
How the record works, what p=none actually does, and the minimum setup that stops your domain from being used to send mail you didn't authorize.
What MX records are, how priorities work, and the five MX mistakes that cause mail to bounce. Check your own in seconds with the free MX checker.
What each record proves, what happens when one is missing, and the order to fix them in. With the exact TXT records to start from.
The three DNS lookups that tell you whether someone can send "from" your domain and get it delivered, and what to do if the answer is yes.
What each policy actually does, when to move between them, and the safe order that avoids rejecting your own transactional mail.
The concrete requirements Gmail enforces for bulk senders, in the order to check them: authentication, alignment, spam rate, unsubscribe, volume.
Authentication, alignment, reputation, list quality, content signals, warmup, and blocklists. The order to check them and how to tell which one you have.
What blocklists are, how to check if your sending IP is listed on Spamhaus, SpamCop, or Barracuda, and how to get delisted fast.
How to check your sending IP against 9 major blocklists, what a listing means for deliverability, and how to get delisted from Spamhaus, SpamCop, and Barracuda.
On the SBL or XBL? Confirm the listing, fix the root cause, submit the delisting request, and verify the re-scan. Plus how to stay off the list.
What a 0-100 deliverability score means, what 90+ / 70-89 / 50-69 / below 50 actually tells you, and the order to fix things to move it up.
What aggregate and forensic DMARC reports are, how to read them, and how to set up monitoring so you catch authentication failures before they hurt your deliverability.
Why your SPF record might not be working, the most common syntax errors (missing v=spf1, missing -all, too many DNS lookups, duplicate records), and how to fix them.
Step-by-step guide to configuring SPF, DKIM, and DMARC for Google Workspace so your email authenticates correctly and reaches the inbox.
Step-by-step guide to configuring SPF, DKIM, and DMARC for Microsoft 365 so your email authenticates correctly and reaches the inbox.
Why Gmail rejects your email with SMTP error 550-5.7.26, and how to fix it by configuring SPF, DKIM, and DMARC correctly.
Why your DKIM signature might not be working, the most common causes (missing record, wrong selector, weak key, misalignment), and how to fix them.
The complete 2026 checklist for email authentication for startups: SPF, DKIM, DMARC, and the other records you need to get into the inbox.
What a PTR record is, why receiving servers require it, what forward-confirmed PTR means, and how to check and fix your sending IP.
A step-by-step warm-up plan: how many days, how many emails per day, how to ramp, and how to know when your domain is ready for real volume.
What each SMTP status code means and how to fix it: 250 accepted, 4xx temporary failures, 550 mailbox unavailable, 554 relay denied, and the codes that predict spam placement.
What a DKIM selector is, how to find yours in DNS, how providers name theirs, how to rotate a key without breaking delivery, and the mistakes that fail your DKIM check.
What email bounce codes mean and how to fix them: hard bounces (550, 5.1.1, 5.7.1), soft bounces (450, 4.2.1, 4.3.0), and the bounce-rate thresholds that get you delisted.
Why SPF records are limited to 10 DNS lookups, how to count them, and how to fix a "too many DNS lookups" permerror by flattening and consolidating your includes.
Outlook returns 550 5.7.1 when it does not trust the sender. The three records to check, the IP reputation test, and the order to fix it.
Outlook is not receiving your email. The four causes in order of likelihood, and the exact check for each one.
Yahoo has some of the strictest authentication requirements of the major providers. What it checks, the records it requires, and the order to fix it.
Yahoo rejects your email with 550 5.7.1. What the code means, the five causes in order of likelihood, and the exact fix for each one.
Microsoft is rejecting your email. The five causes in order of likelihood, the exact check for each one, and the fix that actually works.
Yahoo is not receiving your email. The four causes in order of likelihood, and the exact check for each one.
What counts as a good cold email bounce rate, why it matters for deliverability, and the exact steps to bring a high bounce rate back under control.
How many sending domains you need, how to authenticate each one, warm-up, rotation, and the monitoring that catches a broken record before it burns a domain.
What actually decides whether your app's emails land, how to check your sending domain's SPF, DKIM, DMARC, TLS and IP reputation, and how to catch a broken record before your users stop receiving password resets.
The five reasons mail lands in the spam folder, how to tell which one is yours, and the exact record or change that fixes it. Work through them in order and you will usually find the cause in the first two.
Your SaaS sends transactional and notification email through a provider. When deliverability breaks, your users miss password resets and onboarding emails. Here's how to catch it before your users do.
The domain math for cold email, how to set each one up, when to rotate, and how to keep the pool out of spam. For agencies running cold email at volume.
Shopify, WordPress, WooCommerce, Squarespace, Wix, HubSpot, Salesforce, Mailchimp, or Klaviyo. Each platform has different gotchas. Find the guide for your specific stack.
Shopify merchants lose sales when order confirmations land in spam. The 6 DNS records that decide delivery, how to check them in 30 seconds, and how to monitor them.
WordPress sites lose leads when contact form emails and password resets land in spam. The DNS records that decide delivery, the WordPress-specific gotchas, and how to check your domain in 30 seconds.
WooCommerce stores lose sales when order confirmations and shipping updates land in spam. The DNS records that decide delivery, the WooCommerce-specific gotchas, and how to check your domain in 30 seconds.
Squarespace sites lose leads when contact form emails and password resets land in spam. The DNS records that decide delivery, the Squarespace-specific gotchas, and how to check your domain in 30 seconds.
Wix sites lose leads when contact form emails and password resets land in spam. The DNS records that decide delivery, the Wix-specific gotchas, and how to check your domain in 30 seconds.
HubSpot users lose leads when CRM emails and marketing campaigns land in spam. The DNS records that decide delivery, the HubSpot-specific gotchas, and how to check your domain in 30 seconds.
Salesforce Marketing Cloud users lose leads when campaigns and transactional emails land in spam. The DNS records that decide delivery, the Salesforce-specific gotchas, and how to check your domain in 30 seconds.
Mailchimp users lose engagement when campaigns land in spam. The DNS records that decide delivery, the Mailchimp-specific gotchas, and how to check your domain in 30 seconds.
Klaviyo users lose revenue when campaigns land in spam. The DNS records that decide delivery, the Klaviyo-specific gotchas, and how to check your domain in 30 seconds.
ActiveCampaign users lose engagement when campaigns and transactional emails land in spam. The DNS records that decide delivery, the ActiveCampaign-specific gotchas, and how to check your domain in 30 seconds.
ConvertKit users lose engagement when campaigns and automated emails land in spam. The DNS records that decide delivery, the ConvertKit-specific gotchas, and how to check your domain in 30 seconds.
GetResponse users lose engagement when campaigns and automated emails land in spam. The DNS records that decide delivery, the GetResponse-specific gotchas, and how to check your domain in 30 seconds.
Email deliverability guides organized by audience. Find the guide that matches your use case and fix the DNS records that decide delivery.
What an SPF record is, the five most common mistakes, and how to check yours for free. No signup required.
Run a free deliverability test on your domain. Check SPF, DKIM, DMARC, TLS, PTR and IP reputation. Get a spam-risk score and the exact record to fix.
Run a free inbox placement test on your domain. Check SPF, DKIM, DMARC, TLS, PTR and IP reputation to find out if your email lands in the inbox or spam.
Your email might be landing in spam without you knowing. Here are 5 ways to check, from DNS record lookups to free deliverability tests. No signup required.
Run a free email spam test on your domain. Check SPF, DKIM, DMARC, TLS, PTR and IP reputation to find out your spam risk. Get the exact record to fix.
The 7 best email deliverability tools in 2026, compared. Check SPF, DKIM, DMARC, TLS, PTR and IP reputation. Free options and paid options.
Your SPF record might be broken. Here are the 5 most common SPF mistakes and how to fix them, step by step. No signup required.
Set up DMARC on your domain in 5 steps. Learn what DMARC does, how to choose the right policy, and how to monitor your DMARC reports. No signup required.
Your domain might be on a blocklist. Here's how to check if your domain is on a blocklist, and how to get it delisted. No signup required.
The complete guide to email deliverability for agencies and developers. Learn how to fix SPF, DKIM, DMARC, and IP reputation issues across multiple client domains. Free audit, no account needed.
Your domain can be fully authenticated and still land in spam. What domain reputation is, how mailbox providers build it, how to check yours, and the specific fixes that move it.
Amazon SES accepted your mail but recipients are not seeing it. The five reasons SES deliverability drops, and the exact fix for each one.
Amazon SES is the cheapest way to send email at scale. SendGrid is the easiest. Here is how they compare on deliverability, pricing, and the failure modes that actually matter for your app.
Brevo accepted your mail but recipients are not seeing it. The five reasons Brevo deliverability drops, and the exact fix for each one.
Mailjet accepted your send, so the problem is not the API. It is the receiving mailbox. Here are the five reasons Mailjet mail lands in spam, and the exact DNS and account fixes for each.
SMTP error 550 5.7.1 is a permanent policy rejection. What causes it (blocklisted IP, failed SPF/DKIM/DMARC, spam content) and how to fix it.
SMTP error 550 5.1.1 means the recipient mailbox does not exist. How to tell a hard bounce from a typo, and how to stop sending to dead addresses.
Gmail SMTP error 421 4.7.0 means you are opening too many connections from one IP. How to slow down, warm up, and spread volume to stop the rate limit.
Monitor 25 client domains on one dashboard, white-label the reports, and get daily alerts when a record breaks.
See the agency plan →One REST call returns the same 7-check audit the free tool runs. Bearer auth, JSON in, JSON out. 100 audits/day on Pro.
Read the API docs →Welcome emails, password resets and receipts are transactional. If they land in spam, activation drops. Audit your domain in 30 seconds.
See what breaks →Order confirmations, shipping updates and receipts are transactional. If they land in spam, chargebacks follow. Audit your domain in 30 seconds.
See what breaks →Donor appeals, event invites and volunteer emails are your revenue. If they land in spam, you lose gifts and attendance. Audit your domain in 30 seconds.
See what breaks →Onboarding emails, password resets and receipts are transactional. If they land in spam, activation drops and churn goes up. Audit your domain in 30 seconds.
See what breaks →Prefer not to read the theory?
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