New
Test your email deliverability
Send a test email and get a complete diagnosis of your SPF, DKIM and DMARC authentication in seconds.
Resolve, audit and fix DNS, SPF, DKIM, DMARC and BIMI records from a single dashboard with easy-to-use diagnostics.
Aggregated data since 2025
Alerts sent as soon as a difference or latency anomaly is detected
Three modules at the heart of CaptainDNS to troubleshoot zones, secure authentication and keep campaigns out of spam folders.
Instant A, AAAA, MX, TXT and CNAME resolutions across 18 resolvers with latency and RCODE insights.
Start a lookupInstantly validate your DKIM signatures, verify the DNS public key and diagnose configuration errors blocking your emails.
Check a DKIMHardened parsers, policy suggestions and lookup budgeting to prevent blocks and maximise alignment.
Explore analyzers
Paste raw email headers and get a detailed report on authentication, delivery path and spam verdict.
Get a complete deliverability score and actionable recommendations to fix your SPF, DKIM, DMARC, BIMI and MX records.
CaptainDNS · February 26, 2026
Gmail rejecting your emails with 550 5.7.26 or UsernameCaseMapped? Technical causes, step-by-step diagnosis, and SPF/DKIM/DMARC fixes to resolve these permanent bounces.
CaptainDNS · February 25, 2026
A SERVFAIL after enabling DNSSEC points to a broken chain of trust. This guide covers the five causes, three diagnostic commands, and exact fixes for each scenario.
CaptainDNS · February 24, 2026
Microsoft 365 has supported inbound DANE since 2024. This guide explains how to enable it, configure DNSSEC on your domain, and verify the deployment.
We build CaptainDNS to give marketing and SRE teams complete visibility into DNS configuration and deliverability signals. Every tool explains what must be fixed and why it matters.
Our distributed infrastructure mirrors the reality of your recipients. Know whether resolvers in Frankfurt, Sydney or Montreal see the same answer, and get alerted as soon as a divergence appears.
Stay in control of DNS and deliverability with actionable explanations. Here are the answers to the most common requests.
Try it for free, then upgrade to the plan that matches your volume when you're ready.
DNS Lookup
Choose a DNS record type to search for.
Propagation & diagnostics
Compare resolvers worldwide and inspect returned answers.
Keep history, monitor your zones and automate recurring checks.
Email authentication
Tools to verify and validate your email authentication setup.
Deliverability
Email deliverability audit
Audit MX, SPF, DKIM and DMARC to confirm a domain is ready to send.
Mail header analyzer
Decode participants, authentication results and routing from raw headers.
Email tester
Send a test email and get instant authentication and deliverability feedback.
IP Blacklist Checker
Check if your IP is on email blacklists
Domain Blacklist Checker
Check if your domain is on URI blacklists
SPF Flattener
Flatten SPF records to reduce DNS lookups below the 10-lookup limit.
DKIM Selector Finder
Auto-discover all DKIM selectors for a domain via DNS brute-force.
SMTP/MX Tester
Test SMTP connectivity of your MX servers: banner, STARTTLS, TLS certificate, open relay.
IP tools
Quickly inspect an IP address or your own connection.
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.
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.
Web & Certificates
BIMI logos, VMC/CSR certificates and web page diagnostics.
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.
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.