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

SPF syntax validator

SPF record inspector

SPF record generator

DKIM

DKIM syntax validator

DKIM record check

DKIM generator

DMARC

DMARC syntax validator

DMARC record inspector

DMARC record generator

BIMI

BIMI syntax validator

BIMI record inspector

BIMI record generator

MTA-STS

MTA-STS syntax validator

MTA-STS record inspector

MTA-STS generator

TLS-RPT

TLS-RPT syntax validator

TLS-RPT record inspector

TLS-RPT generator

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.

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.

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.

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.

Google Workspace

Google Workspace email setup, DNS records, and admin configuration.

2 articles

February 2026

Configure MTA-STS for Microsoft 365, Google Workspace, and Cloudflare

CaptainDNS · February 8, 2026

Configure MTA-STS for Microsoft 365 and Google Workspace

Step-by-step tutorial to deploy MTA-STS on Microsoft 365, Google Workspace, and Cloudflare. MX patterns, policy file, hosting, and validation included.

December 2025

Diagram showing Gmail stopping POP fetching from external accounts in 2026

CaptainDNS · December 5, 2025

Gmail ends POP fetching from other accounts in 2026: impact and action plan

Starting January 2026, Gmail will no longer continuously fetch messages from external mailboxes via POP ("Check mail from other accounts"). Impacts, timeline, and alternatives for Gmail and Google Workspace users and admins.