DNS & Email Deliverability Guides

Guides for DNS records, propagation delays, and email deliverability — NXDOMAIN, SPF/DKIM/DMARC, MX, nameservers, and blacklists. Each guide includes a free check for your domain.

DNS — 93 guides

421 4.7.28: Fix Gmail's 'Unusual Rate' Deferral

421 4.7.28: Gmail is deferring your mail over an unusual rate on your IP or domain. Find which identity it's throttling. Free instant check, no sign-up.

Diagnose

450 4.2.1 Receiving Mail Too Quickly (Gmail)

450 4.2.1 from Gmail: the recipient is receiving mail too fast — a temporary, retryable limit. Know when to wait and when it's your relay. Free instant check.

Diagnose

451 4.7.1 Please Try Again Later (Greylisting)

451 4.7.1 'try again later' is usually greylisting, not a bounce. See when it clears and when IP reputation is the real cause. Free instant check, no sign-up.

Diagnose

452 4.2.2 Mailbox Over Quota: Temporary, Not a Bounce

452 4.2.2: the recipient's mailbox is temporarily full — a soft bounce your server retries for days, not a hard rejection. Free instant check, no sign-up.

Diagnose

452 4.5.3 Too Many Recipients: Why Retrying Won't Help

452 4.5.3 Too many recipients: one message passed the recipient cap. Retrying won't help — split the RCPT list into chunks. Free mail check, no sign-up.

Diagnose

523 Origin Is Unreachable (Cloudflare) Fix

Cloudflare 523 means Cloudflare can't route to your origin's IP. Fix it in 3 checks: stale A record, dead origin, routing. Free instant check, no sign-up.

Diagnose

535 5.7.8 Username and Password not accepted

535 5.7.8 Username and Password not accepted means SMTP login failed. Fix it: app password, 2-Step Verification, or OAuth. Free instant check, no sign-up.

Diagnose

550 5.1.1 User Unknown: Recipient Doesn't Exist

550 5.1.1 User Unknown means the mailbox doesn't exist, not SPF. Pin the cause in 3 checks: the address, MX routing, aliases. Free instant check, no sign-up.

Diagnose

550 5.1.10 Recipient Not Found (Exchange Online)

550 5.1.10 Recipient not found isn't SPF — the address isn't in the directory. Fix it in 3 checks: address, hybrid sync, MX. Free instant check, no sign-up.

Diagnose

550 5.4.1 Recipient Address Rejected: Access Denied

550 5.4.1 Recipient Address Rejected means a directory block, not SPF. Pin the cause in 3 checks: recipient, DBEB, relay. Free instant check, no sign-up.

Diagnose

550 5.4.5 Daily User Sending Quota Exceeded

550 5.4.5 means Gmail hit your daily send limit and blocked outbound mail. See which cap you crossed, when it resets, and how to raise it. Free email check, no sign-up.

Diagnose

550 5.7.1 This Message Is Not RFC 5322 Compliant

550 5.7.1 not RFC 5322 compliant means a duplicate or malformed header — two Date, From, or Message-ID lines. Find it in the raw source. Free mail check.

Diagnose

550 5.7.1 Unable to Relay: Fix the Exchange Bounce

550 5.7.1 Unable to Relay means Exchange refused to forward your mail. Fix it in 3 checks: auth, connector IP, accepted domain. Free instant check, no sign-up.

Diagnose

550 5.7.1: Fix SPF and Policy Rejection Bounces

550 5.7.1 means the receiver permanently refused your mail on policy — usually SPF. Pin down which of 4 causes in 5 checks. Free instant check, no sign-up.

Diagnose

550 5.7.23: Fix the SPF Check Failed Bounce

550 5.7.23 means a receiver rejected your mail because its sending IP failed your SPF record. Fix it in 3 checks: SPF, the IP, and -all. Free instant check.

Diagnose

550 5.7.25: Fix Gmail's Reverse DNS (PTR) Rejection

550 5.7.25: Gmail rejected your mail — the sending IP's PTR is missing or mismatched. Check PTR, forward match, and FCrDNS. Free instant check, no sign-up.

Diagnose

550 5.7.26: Fix Gmail's Unauthenticated Email Block

550 5.7.26 means Gmail blocked your mail as unauthenticated. Fix it in 4 checks: SPF, DKIM, DMARC, and From alignment. Free instant check, no sign-up.

Diagnose

550 5.7.350 Remote Server Returned Message Detected as Spam

550 5.7.350 means a recipient server flagged your outbound mail as spam. Fix it in 3 checks: SPF, DKIM, and DMARC on your sending domain. Free instant check.

Diagnose

550 5.7.509: Fix Microsoft's DMARC Reject Bounce

550 5.7.509 means Microsoft enforced your own p=reject policy on unaligned mail. Fix alignment in 4 checks: SPF, DKIM, DMARC. Free instant check, no sign-up.

Diagnose

550 5.7.511 Access denied, banned sender

550 5.7.511 means Microsoft 365 banned your sending IP — its strongest block, not self-service. What to fix, then how to appeal. Free instant check, no sign-up.

Diagnose

550 5.7.515 Access Denied (Outlook.com)

550 5.7.515 means Outlook.com rejected your mail: the From domain isn't authenticated. Check SPF, DKIM, DMARC alignment. Free instant check, no sign-up.

Diagnose

552 5.2.2 Mailbox Full / Over Quota: Fix

552 5.2.2: the recipient's mailbox is over quota, not your SPF or DKIM. Tell permanent from temporary and who must fix it. Free instant check, no sign-up.

Diagnose

553 5.1.8 Domain of Sender Address Does Not Exist

553 5.1.8 means your envelope-sender domain doesn't resolve — not the recipient. Check the sender domain's A/MX in 3 steps. Free instant check, no sign-up.

Diagnose

554 5.4.14 Hop Count Exceeded (Mail Loop): Fix

554 5.4.14 Hop count exceeded means your mail is looping. Trace the loop via Received headers and MX routing, then fix it. Free instant check, no sign-up.

Diagnose

554 5.7.1 Relay Access Denied: Fix the Bounce

554 5.7.1 Relay Access Denied means a server refused to forward your mail. Find why in 3 checks: auth, MX, allowed senders. Free instant check, no sign-up.

Diagnose

A Record: Point Your Domain to a Server IP

Point a domain to a server IP with an A record: apex vs subdomain, the host-field trap that doubles your name, and why it won't resolve yet. Free instant check, no sign-up.

Diagnose

A/AAAA Record Check: dig, Broken IPv6 and Multiple IPs

How to check A and AAAA records with dig, tell NXDOMAIN from 'no A record', find an AAAA that points nowhere, and fix stale or extra IPs after a move.

Diagnose

Cafe24 Domain Not Connecting? Fix DNS & Nameservers

Cafe24 domain not connecting after a nameserver change? Check registrar NS, A/CNAME records, and propagation in 4 steps to find the broken layer. Free instant check.

Diagnose

CDN Cache or DNS Propagation? How to Tell Which Is Stale

Old content after a change? Check whether DNS still points at the old server or a CDN is serving a cached copy, using dig, curl --resolve and cache headers.

Diagnose

Cloudflare Nameservers Not Updating? 4-Step Fix

Cloudflare nameservers stuck pending or not updating? Change them at your registrar, clear old DNSSEC, and verify delegation in 4 steps. Free instant check.

Diagnose

CNAME vs A Record: Differences and When to Use Each

An A record maps a name to an IP; a CNAME makes it an alias of another name. When to use each, why a CNAME can't sit on the root domain, and what to do instead.

Diagnose

curl: (6) Could not resolve host: Fix DNS

curl: (6) Could not resolve host means name resolution failed before any connection. Isolate a typo, resolver, or proxy env. Free instant check, no sign-up.

Diagnose

DKIM Fail and "Body Hash Did Not Verify": Find the Cause

Read the Authentication-Results line, tell a body hash failure from a bad signature or a missing key, and fix the step that changed your message or key.

Diagnose

DKIM Record Setup and Check (DNS TXT)

DKIM record setup and check: publish the selector._domainkey TXT, avoid the two-record split trap, and confirm the key resolves. Free instant check, no sign-up.

Diagnose

DKIM Selector Not Found: dkim=none and Missing Key

DKIM selector not found means receivers can't fetch your key. Check the s= selector, CNAME delegation, and typos in 4 steps. Free instant check, no sign-up.

Diagnose

DMARC p=none vs quarantine vs reject, and When to Switch

What DMARC p=none, quarantine and reject do to failing mail, why scanners flag p=none, and what to check in aggregate reports before tightening the policy.

Diagnose

DMARC Record Setup: A Correct Example and How to Check It

DMARC record setup, done right: the exact tags to publish at _dmarc, why pct is gone in 2026, and how to verify it actually resolves. Free instant check, no sign-up.

Diagnose

DNS Propagation Time: How Long It Takes and Why

A DNS change shows up as cached answers expire: at most the old TTL for a record, up to 48 hours for a nameserver change. How to check it and how to shorten it.

Diagnose

DNS REFUSED: Fix the Policy Block Response

DNS REFUSED means a server won't answer you on purpose — wrong nameserver, no recursion, or an ACL. Find which in 4 checks. Free instant check, no sign-up.

Diagnose

DNS Server Not Responding (Windows): Fix

DNS server not responding on Windows? Isolate resolver vs website: flush DNS, switch to a public resolver, reset the adapter. Free instant check, no sign-up.

Diagnose

DNS SERVFAIL: Fix Server Failure Responses

DNS SERVFAIL means the resolver tried and failed — usually DNSSEC or a dead nameserver. Split the cause in 4 checks. Free instant check, no sign-up.

Diagnose

DNS TTL Best Practices: What Values to Use and Why

What a DNS TTL really controls, sensible values (300, 600, 3600, 14400, 86400), the maximum and minimum, negative caching, and how to lower TTL before a move.

Diagnose

DNS_PROBE_FINISHED_BAD_CONFIG Fix

DNS_PROBE_FINISHED_BAD_CONFIG in Chrome? Confirm the domain resolves publicly, then fix the local resolver, cache, or router. Free instant check, no sign-up.

Diagnose

DNS_PROBE_FINISHED_NO_INTERNET Fix

DNS_PROBE_FINISHED_NO_INTERNET in Chrome? Confirm the site resolves publicly, then isolate your network, resolver, or VPN. Free instant check, no sign-up.

Diagnose

DNS_PROBE_FINISHED_NXDOMAIN: What It Means and How to Fix It

DNS_PROBE_FINISHED_NXDOMAIN means DNS says the name doesn't exist. How to tell your device from the domain, read dig output, and fix each real cause.

Diagnose

DNSSEC Validation Failed: Fix the SERVFAIL

DNSSEC validation failed? SERVFAIL from expired RRSIG, a missing DS, or a bad key rollover. Find the broken link in 5 checks. Free instant check, no sign-up.

Diagnose

Dothome Domain Not Connecting? Fix DNS & Nameservers

Dothome domain not connecting after a nameserver change? Check registrar NS, A/CNAME, and propagation in 4 steps to find the broken layer. Free instant check.

Diagnose

Email Deliverability Setup: SPF, DKIM, DMARC, Sender Rules

Set up SPF, DKIM and DMARC in the right order, meet the Gmail, Yahoo and Microsoft bulk-sender rules, and find out why authenticated mail still lands in spam.

Diagnose

Email Forwarding Setup: Why It Breaks SPF and How to Check

Email forwarding setup breaks SPF the instant you turn it on. Check three failure points — SPF, DKIM, loops — in order. Free instant check, no sign-up.

Diagnose

Emails Going to Spam: Fix Deliverability

Emails landing in spam instead of the inbox? Fix it in 5 checks: SPF/DKIM alignment, DMARC, sending IP reputation, and PTR. Free instant check, no sign-up.

Diagnose

ERR_ICANN_NAME_COLLISION: Fix the Chrome DNS Error

ERR_ICANN_NAME_COLLISION means a name resolved to 127.0.53.53. Check the name, your search domain, and if the TLD is public. Free instant check, no sign-up.

Diagnose

ERR_NAME_NOT_RESOLVED: What It Means and How to Fix It

ERR_NAME_NOT_RESOLVED means DNS gave no usable address for a hostname. How to tell NXDOMAIN from SERVFAIL with dig, and fix DNSSEC, expiry, Chrome DNS and more.

Diagnose

Error 1000 DNS Points to Prohibited IP (Cloudflare)

Error 1000: your Cloudflare DNS record points at a Cloudflare IP, not your origin. Find the looping A record in 3 checks. Free instant check, no sign-up.

Diagnose

Error 1001 DNS Resolution Error (Cloudflare): Fix

Error 1001 means Cloudflare can't map the requested hostname to a zone. Fix in 3 checks: the DNS record, a dangling Cloudflare IP, propagation. Free instant check, no sign-up.

Diagnose

Error 1014 CNAME Cross-User Banned (Cloudflare)

Error 1014 means your CNAME crosses two Cloudflare accounts and is banned. Check the target, swap to an A record, or use SaaS. Free instant check, no sign-up.

Diagnose

Error 1016 Origin DNS Error (Cloudflare): Fix

Error 1016 means Cloudflare can't resolve your origin's IP. Fix in 3 checks: A/AAAA record, CNAME target, origin host. Free instant check, no sign-up.

Diagnose

Error 1018 Unable to Resolve (Cloudflare): Fix

Error 1018 means Cloudflare can't apply your zone's config at the edge — a recent activation or a ghost partner zone, not a record you fix. Free instant check, no sign-up.

Diagnose

Error 530 (Cloudflare): Read the 1xxx Code Inside

Error 530 from Cloudflare is a wrapper, not the error. Find the 1xxx code in the page body, decode it, and fix the origin. Free instant check, no sign-up.

Diagnose

Find Your Domain's Nameservers and DNS Provider

Find your domain's nameservers to see who hosts your DNS and where to edit records. Check the authoritative NS in seconds. Free instant check, no sign-up.

Diagnose

Gabia Domain Not Connecting? Fix Nameservers & DNS

Gabia domain not connecting after a nameserver change? Check registrar NS, DNS zone, and propagation in 4 steps to find the broken layer. Free instant check.

Diagnose

getaddrinfo EAI_AGAIN: Fix the Node.js DNS Timeout

getaddrinfo EAI_AGAIN means Node's resolver timed out — a transient failure, not a dead name. Tell it apart from a real bug in 3 checks. Free instant check, no sign-up.

Diagnose

getaddrinfo ENOTFOUND: Fix the Node.js DNS Error

getaddrinfo ENOTFOUND means Node couldn't resolve the hostname. Tell a dead domain from a typo or full-URL host in 3 checks. Free instant check, no sign-up.

Diagnose

GitHub Pages Custom Domain: Setup and DNS Check

GitHub Pages custom domain not connecting? Set apex A/AAAA and www CNAME right, then fix the apex-CNAME and stuck-HTTPS traps. Free instant check, no sign-up.

Diagnose

GoDaddy Nameserver Change Not Working? 4-Step Fix

GoDaddy nameserver change not taking effect? Check the registrar NS, DNS zone, and propagation in 4 steps to find the broken layer. Free instant check.

Diagnose

Google Workspace Domain Setup: Verify TXT, Then MX

Google Workspace domain setup in the right order: verify a TXT record, point MX to smtp.google.com, then add SPF and DKIM. Free instant check, no sign-up.

Diagnose

hanmail.net / Daum Mail Rejected: Fix the Bounce

Mail to daum.net or hanmail.net bouncing or blocked? Check SPF, reverse DNS, and Daum's IP unblock path in 3 steps to find why. Free instant check.

Diagnose

How to Add a CNAME Record (Setup and Check)

Add a CNAME record: point one name at another, dodge the apex and 'no other records' traps, and confirm it resolves. Free instant check, no sign-up.

Diagnose

How to Check MX, SPF, DKIM and DMARC Records

The dig and nslookup commands that show a domain's MX, SPF, DKIM and DMARC records, what a healthy answer looks like, and the mistakes to look for in each.

Diagnose

How to Create a Subdomain (CNAME or A Record)

Create a subdomain: choose A vs CNAME, avoid the host-field trap that doubles your domain, and see why it won't resolve yet. Free instant check, no sign-up.

Diagnose

Imweb Custom Domain Not Connecting? Fix DNS

Imweb custom domain not connecting? Nameserver, CNAME, or forwarding — each fails differently, and only one gets you SSL. Free instant check, no sign-up.

Diagnose

Mail Server Blacklisted: How to Check, Fix and Get Delisted

How to check which DNSBL lists your mail IP, read the return codes, find the real cause (compromised account, script, open relay) and request delisting.

Diagnose

Microsoft 365 Domain Setup: TXT, MX, SPF, DKIM

Microsoft 365 domain setup in the right order: verify a TXT record, point MX to Exchange Online, then SPF and DKIM. Free instant check, no sign-up.

Diagnose

MTA-STS and TLS-RPT Setup: Enforce SMTP TLS

MTA-STS setup: publish the TXT, host the policy over HTTPS, add TLS-RPT, and start in testing before you enforce. Free instant check, no sign-up.

Diagnose

MX Record Setup and Check: Priority and Failover

MX records route your inbound mail. Verify priority numbers, backup MX failover, the CNAME trap, and split-MX in 5 checks. Free instant check, no sign-up.

Diagnose

Nameserver Change: How to Check It Took Effect

Changed your nameservers and not sure it took? Check TLD delegation and resolver cache to tell 'still propagating' from actually broken. Free instant check.

Diagnose

Nameserver Mismatch: Registry vs Zone NS, Lame Delegation

Compare the NS records at the registry with what each nameserver answers, find lame or stale servers, and fix delegation and DS after a DNS provider move.

Diagnose

Naver Mail Rejected as Spam: Fix Bounced Delivery

Naver mail bouncing as spam or blocked? Check SPF, DKIM alignment, and KISA white-domain status in 3 steps to find why Naver rejects you. Free instant check.

Diagnose

Naver Works Mail Not Receiving: Fix MX, SPF, DKIM

Naver Works mail not arriving or landing in spam? Fix it in 4 checks: MX host, split MX, SPF include, DKIM selector. Free instant check, no sign-up.

Diagnose

NET::ERR_ADDRESS_UNREACHABLE: No Route to Host

NET::ERR_ADDRESS_UNREACHABLE means there is no network route to the resolved IP, usually a broken IPv6 AAAA record. Check A vs AAAA and reachability in 3 steps. Free instant check.

Diagnose

No MX Record: Email Not Delivering

No MX record means inbound mail has nowhere to land. Check MX presence, priority, the A-record fallback, and null MX in 5 steps. Free instant check, no sign-up.

Diagnose

Reverse DNS (PTR) Setup: Who Controls It and How to Set It

Set up reverse DNS for a mail server: why your DNS host can't, finding who controls the PTR, setting it on AWS, Azure and others, and checking FCrDNS.

Diagnose

Shopify Custom Domain Not Connecting

Shopify custom domain not connecting: fix it in 3 checks — apex A record vs www CNAME, DNS propagation, and automatic SSL. Free instant check, no sign-up.

Diagnose

SMTP Port 25 Blocked: Why It Happens and the Fix

SMTP port 25 blocked usually means your ISP or cloud host bars outbound mail. Tell the cases apart and move to 587/465 in 4 checks. Free instant check, no sign-up.

Diagnose

SPF PermError: Too Many DNS Lookups (Over 10) Fixed

Why SPF returns permerror past 10 DNS lookups, which terms count, how to count nested includes yourself, and the fixes that actually get you back under.

Diagnose

SPF Record: Check and Set It Up Correctly

SPF record check and setup: the three mistakes that break it — two records, +all, the 10-lookup limit — and how to fix each. Free instant check, no sign-up.

Diagnose

SRV Record Setup: Priority, Weight, Port, Target

SRV record setup: the _service._proto name, priority, weight, port, and target — plus why the target can't be a CNAME. Free instant check, no sign-up.

Diagnose

Subdomain Takeover: Find Dangling DNS Records

Subdomain Takeover: a CNAME points to a deprovisioned cloud resource anyone can re-register. Spot dangling DNS in 4 checks. Free instant check, no sign-up.

Diagnose

Tistory Custom Domain Setup: CNAME, apex & HTTPS

Tistory custom domain not connecting? CNAME to host.tistory.io, apex needs ALIAS, unify www vs root so HTTPS issues. Free DNS check, no sign-up.

Diagnose

TXT Record for Domain Ownership Verification

TXT record for domain ownership verification: where the token goes, root vs host, and the deletion trap that unverifies you. Free instant check, no sign-up.

Diagnose

TXT Record Over 255 Characters: Split It Right

TXT record over 255 characters fails because each string caps at 255 bytes. Split it into quoted chunks the right way in 4 checks. Free instant check, no sign-up.

Diagnose

Vercel Custom Domain Not Connecting

Vercel custom domain not connecting: fix it in 3 checks — apex A record vs www CNAME, DNS propagation, and automatic SSL. Free instant check, no sign-up.

Diagnose

Wildcard DNS Record Setup: What * Actually Matches

Wildcard DNS (*) not catching every subdomain? See what * matches, why one existing record can block it, and the SSL limit. Free instant check, no sign-up.

Diagnose

Wix Custom Domain Not Connecting? Fix It in 4 Steps

Wix custom domain not connecting? Pick pointing vs name servers, set the records Wix shows, and check propagation and SSL in 4 steps. Free instant check.

Diagnose
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