Tools
Everything here
Domain report
Delegation, registration and expiry, DNSSEC, SPF with the include tree followed, DMARC, DKIM and mail transport security.
Email header analyser
Every hop a message took, how long each waited, and whether SPF, DKIM and DMARC passed. Parsed in your browser.
Subnet calculator
Network and broadcast addresses, netmask, wildcard, usable range and host count for any IPv4 CIDR block.
DNS record reference
What each record type does, what its fields mean, and the mistake people make with it.
TLS certificates
Every certificate ever issued for a domain, from the public Certificate Transparency logs — expiry dates, issuing authorities, and every hostname that has appeared in one.
Reverse DNS lookup
Find the PTR record for an address, and check that it agrees with the forward record — the thing mail receivers actually test.