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CaptainDNS

Email Diagnostics

Tools to verify and validate your email authentication setup.

SPF
DKIM
DMARC
DMARCbis
BIMI
MTA-STS
TLS-RPT
DANE / TLSA

Deliverability

Email deliverability auditMail header analyzerEmail testerIP Blacklist CheckerDomain Blacklist CheckerSPF FlattenerDKIM Selector FinderSMTP/MX Tester

Secure & Monitor

Record generators, policy hosting and continuous monitoring.

Network & Web

Network tools, web page analysis and certificates.

IP Tools

My IP address

Detect your IPv4/IPv6 addresses and their geolocation.

Reverse lookup

Resolve a PTR record and validate DNS consistency.

IP WhoIs

Identify the owner of an IP range and its contacts.

IPv4 netmask

Calculate network, broadcast and usable hosts for any IPv4 block.

IPv6 subnet calculator

Calculate IPv6 subnets, address ranges and reverse DNS entries.

Certificates & BIMI

BIMI logo lookup

Inspect a BIMI logo URL - format, metadata and live rendering - before rollout.

BIMI SVG converter

Convert any SVG to BIMI-compliant Tiny-PS format in seconds.

CSR parser

Inspect a CSR, extract subject details, fingerprints and requested SANs.

VMC inspector

Inspect a Verified Mark Certificate - issuer, validity and SAN coverage - before you publish BIMI.

Web

Page size checker

Check if your page exceeds Google's 2 MB crawl limit.

Phishing URL Checker

Check if a URL is flagged as phishing or malware by 4 threat intelligence sources.

Redirect Checker

Analyze HTTP redirect chains

Domain Redirect

Redirect your domains with automatic HTTPS and 301/302 redirects.

Developer Tools

Text utilities and tools for everyday dev work.

Text

Transform and measure your content in seconds.

Text case converter

Convert any block of text to upper or lower case instantly.

Slug generator

Transform any sentence into an SEO-friendly slug in seconds.

Word & character counter

Measure the length of any text, with instant word and character counts.

Password generator

Generate strong random passwords and memorable passphrases instantly.

Developer

Encoding, hashing, regex and formatting for everyday dev work.

Base64 encoder / decoder

Encode or decode any content in Base64 without leaving the browser.

Hash Generator

Compute MD5, SHA-1, SHA-256 and SHA-512 hashes of any text.

URL encoder / decoder

Encode or decode text using percent-encoding (RFC 3986) right in your browser.

Regex Tester

Test a regular expression against text and visualize matches.

JSON / YAML Formatter

Format, validate and convert JSON and YAML in one click.

CaptainDNS hosts your MTA-STS policy and BIMI logo, and monitors your DMARC and TLS-RPT reports automatically. All free, no server required.

Google, Yahoo and Microsoft now require stronger email authentication. Protect your deliverability in just a few clicks.

MTA-STSBIMITLS-RPTDMARC Monitoring

DANE

DANE guides and tutorials: authenticate TLS certificates via DNS using DNSSEC and TLSA records.

7 articles

March 2026
February 2026