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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 9, 2026
Your MTA-STS policy is unreachable, the TLS certificate is rejected, or enforce mode is blocking emails? This troubleshooting guide covers common errors and their solutions.
CaptainDNS · February 8, 2026
Step-by-step tutorial to deploy MTA-STS on Microsoft 365, Google Workspace, and Cloudflare. MX patterns, policy file, hosting, and validation included.
CaptainDNS · February 6, 2026
MTA-STS (RFC 8461) enforces TLS encryption for incoming emails. Learn how to configure it, choose between testing and enforce modes, and validate your 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
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.
Images
Preview and validate your BIMI logos before publishing.
Certificates
Inspect CSRs, BIMI logos and VMC certificates before you deploy.
IP
Look up addresses, owners, reverse records and ranges.