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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 14, 2026
A practical guide to reading and leveraging TLS-RPT reports: JSON structure, failure types, diagnosing certificate and STARTTLS errors, and a daily analysis workflow.
CaptainDNS · February 13, 2026
Hands-on tutorial to set up TLS-RPT (SMTP TLS Reporting) on Microsoft 365, Google Workspace, and OVHcloud. DNS record, verification, and troubleshooting included.
CaptainDNS · February 12, 2026
The median web page weight has grown 5x in 15 years. HTTP Archive data, resource breakdown by type, SEO impact, and projections for 2026-2028.
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.
Text
Transform, encode and measure your content in seconds.
Text case converter
Convert any block of text to upper or lower case instantly.
Base64 encoder / decoder
Encode or decode any content in Base64 without leaving the browser.
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.
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.
IP
Look up addresses, owners, reverse records and ranges.