CaptainDNS

DNS Lookup

Choose a DNS record type to search for.

Propagation & diagnostics

Compare resolvers worldwide and inspect returned answers.

Propagation testPublic resolvers, remaining TTLs, divergent responses.
Domain DNS auditParent/local NS, authority and response times.
API & integrations
Keep history, monitor your zones and automate recurring checks.

Email authentication

Tools to verify and validate your email authentication setup.

Syntax validation

Record inspectors

Generators

SPF syntax validator

Review include chains and DNS query limits before publishing changes.

SPF record inspector

Resolve the published TXT record, flag lookup limits and decode each mechanism.

SPF record generator

Generate SPF records with pre-configured email providers.

DKIM syntax validator

Quickly check the syntax of your DKIM records.

DKIM record check

Resolve a selector and analyse the published DKIM TXT record.

DKIM generator

Generate DKIM key pairs (RSA/Ed25519) and DNS TXT records.

DMARC syntax validator

Check rua/ruf destinations, alignment modes and the active policy before publishing.

DMARC record inspector

Resolve the published policy, surface diagnostics and confirm enforcement before switching to reject.

DMARC record generator

Create compliant DMARC records with all policy and reporting options.

BIMI syntax validator

Validate BIMI tags, the logo fetch and the VMC certificate before publishing.

BIMI record inspector

Resolve the published BIMI record, inspect logo and VMC diagnostics in one place.

MTA-STS syntax validator

Validate MTA-STS TXT records and policy files offline.

MTA-STS record inspector

Check MTA-STS configuration including DNS, policy, and TLS.

MTA-STS generator

Generate MTA-STS DNS records and policy files.

TLS-RPT syntax validator

Validate TLS-RPT record syntax offline before publishing.

TLS-RPT record inspector

Look up and validate SMTP TLS Reporting for any domain.

TLS-RPT generator

Generate TLS-RPT DNS records with reporting URIs.

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.

BIMI logo lookup

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

Certificates

Inspect CSRs, BIMI logos and VMC certificates before you deploy.

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.

Look up addresses, owners, reverse records and ranges.

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.

My IP address

Detect your IPv4/IPv6 addresses and their geolocation.

MTA-STS

MTA-STS guides and tutorials: secure email transport with mandatory TLS encryption.

1 article

February 2026

MTA-STS: securing email transport with mandatory TLS encryption

February 6, 2026

MTA-STS: the complete guide to securing your email transport

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.