Domain Tools
Domain-level questions: who registered it and when, when does it expire, where does the URL actually end up after redirects, and what the server tells browsers through its headers.
Diagnostic order
All domain tools
Domain Checkup
Everything at once: DNS, MX, SPF, DKIM, DMARC, DNSSEC, SSL, expiry
WHOIS Lookup
Registrar, dates, and nameservers for a domain
Domain Age Checker
How old is a domain, and when does it expire?
Redirect Checker
Follow a URL through every redirect hop
HTTP Headers Checker
Response headers a server actually sends
Security Headers Checker
HSTS, CSP, and friends — graded