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 — 26 guides

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.

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.

A/AAAA Record Troubleshooting Checklist

A record not resolving? Fix missing or wrong A/AAAA records in 5 steps: values, TTL, delegation, propagation, IPv6. Free instant DNS check, no sign-up.

CDN Cache vs DNS Propagation

Changes not visible after a DNS update? Check resolver answers first, then CDN cache headers, then purge or wait out TTL. Free instant check, no sign-up.

CNAME vs A Record — When to Use Which

CNAME vs A record: what each does, when to use which, and the apex domain rule that breaks naive setups. Free instant DNS lookup, no sign-up.

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.

DKIM Verification Failed: dkim=fail and Body Hash Mismatch

Why a correctly signed email still fails DKIM — body hash mismatch, missing selector, revoked key, and DMARC alignment — and how to find the real cause.

DMARC none vs quarantine vs reject: Which Policy to Use

DMARC none vs quarantine vs reject: what each policy does and a 3-stage rollout to reach reject safely. Free instant DMARC record check, no sign-up.

DNS Propagation Delay: Why Changes Take Time to Appear

DNS propagation delay or cache? Separate TTL, resolver cache, and CDN caching in 3 checks across regions. Free instant propagation check, no sign-up.

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.

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.

DNS TTL Best Practices — Optimal Values for Every Scenario

DNS TTL best practices: recommended values for stable records, failover, and migrations. Free instant DNS check shows your current TTLs, no sign-up.

DNS_PROBE_FINISHED_NXDOMAIN Fix Checklist

DNS_PROBE_FINISHED_NXDOMAIN? Fix it in 4 checks: DNS records, nameserver delegation, resolver cache, domain status. Free instant check, no sign-up.

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.

Email Going to Spam? SPF, DKIM, and DMARC Setup

Emails going to spam? Set up SPF, DKIM, and DMARC in 3 steps with copy-paste DNS records. Free instant check shows what's missing, no sign-up.

ERR_NAME_NOT_RESOLVED Fix

ERR_NAME_NOT_RESOLVED means DNS couldn't turn the hostname into an IP. Tell a dead domain from a broken resolver in 3 checks. Free instant check, no sign-up.

Mail Server Blacklist Removal Checklist

Mail server blacklisted? Run 3 checks before requesting delisting: sending hygiene, abuse review, RBL-specific cleanup. Free instant check, no sign-up.

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.

MX/SPF/DMARC Basic Checks

Email not delivering? Check MX, SPF, DKIM, and DMARC in 4 steps to separate delivery failures from spam filtering. Free instant check, no sign-up.

Nameserver Mismatch Diagnosis

Nameserver mismatch breaks DNS silently. Compare registrar NS records with the zone's actual servers and fix delegation. Free instant check, no sign-up.

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.

Reverse DNS / PTR Setup Guide

PTR record missing? Mail servers may reject you. Set reverse DNS with your IP provider and match your sending hostname. Free instant check, no sign-up.

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.

SPF Permerror: Too Many DNS Lookups

SPF permerror? Get under the 10 DNS lookup limit in 3 steps: count lookups, flatten includes, drop dead mechanisms. Free instant check, no sign-up.

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.

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.