Six free tools for DNS and email security.
Look up records, validate syntax, generate policies. Every tool runs without a login, ten checks per tool per day, and returns the exact DNS value you need to paste.
Or browse the six tools below. Nothing is stored server-side.
DNS Health Check
Map SPF, DKIM, DMARC, and DNSSEC records visually. Instant DNS security audit.
dig ANY example.comOpen →DMARC Checker & Generator
Check your DMARC record or generate a new policy to prevent email spoofing.
dig TXT _dmarc.example.comOpen →SPF Record Checker
Validate SPF syntax, detect excessive lookups and missing -all policy.
dig TXT example.com | grep spf1Open →MTA-STS Validator
Verify your MTA-STS DNS record and policy file for encrypted email delivery.
curl mta-sts.example.com/.well-knownOpen →Bulk DNS Checker
Look up A, AAAA, MX, NS, CNAME, TXT, SOA, CAA records for multiple domains.
dig MX example.com +shortOpen →Email ROI Calculator
Calculate revenue lost to poor email deliverability and spam folders.
sent × bounce_rate × valueOpen →Quick fixes
The five errors these tools find most often.
Each guide states the cause in one paragraph, then three steps that end in a re-test. No sign-up wall.
Fix Gmail 550 5.7.26 Authentication Required
Gmail rejected your message with 550 5.7.26 authentication required. Fix SPF, DKIM, DMARC alignment, and envelope-from settings in three steps.
Fix SPF PermError (Permanent Error)
SPF PermError means your SPF record is invalid or exceeds 10 DNS lookups. Remove dead includes, fix syntax, and validate before mail fails DMARC.
Fix DMARC Alignment Failed (SPF Passed)
DMARC alignment failed but SPF passed? Your envelope-from or DKIM domain does not match the visible From address. Diagnose and fix alignment in three steps.
Fix Gmail Unverified Sender | DMARC Guide
Gmail unverified sender means missing DMARC or failed alignment. Publish DMARC, enable DKIM on M365 or ESP paths, and validate with the DMARC checker.
Fix SPF Too Many DNS Lookups
SPF records allow max 10 DNS lookups. Too many includes cause PermError and deliverability failures. Audit includes and validate with the SPF checker.
Free tool or continuous monitoring?
The free tools answer a question today. Monitoring answers it every morning, for every domain you own.
Free tools
10 checks per tool, per day
- ·One domain at a time, on demand
- ·Record syntax, lookups and policy parsing
- ·Copy-paste DNS values
- ·Fix guide for every finding
- ·No account, no history
Monitoring
From $29/mo · 7-day trial
- ·Daily audits across every domain
- ·35 checks including DNSSEC, MTA-STS, BIMI
- ·Email and Slack alerts on drift
- ·Subdomain takeover discovery
- ·PDF and CSV evidence for auditors
DNS & email reference
Long-form error codes, fix walkthroughs, and provider DNS tables live on our public reference site. These pages are written for search and for engineers pasting records into Cloudflare or M365.
- Gmail 550 5.7.26, error code
- Fix Gmail 550 5.7.26
- SPF PermError
- Fix SPF too many lookups
- DMARC alignment failed
- What is DMARC
- What is SPF
- NIS2 Article 21
- Microsoft 365 DNS
- Google Workspace DNS
Hosted on security.zerohook.org. RFC-grounded reference pages (error codes, fixes, NIS2, provider DNS).
Need continuous monitoring?
ZeroHook automatically monitors your DNS configuration 24/7 and alerts you to changes, expiring certificates, and security issues before they affect your business.