Email and DNS tools
The records that decide whether your mail arrives, and whether anything resolves.
SPF, DKIM and DMARC are single TXT records, and all three fail in ways that look like success. SPF has a hard limit of ten DNS lookups counted recursively through every include, and exceeding it is a permerror, which means receivers ignore the record entirely: it stops authorising your own mail as well as blocking anyone else's. A DKIM record with an empty p is not incomplete, it is a revocation. DMARC at p=none reports and enforces nothing, which is the correct place to start and where most domains stop. Underneath all three is the zone itself, where a target missing its trailing dot silently becomes a name that does not exist. These tools read the record and name what it actually does.
Free, with no sign-up. Every one runs entirely in your browser: nothing you paste is uploaded, and there is no server to send it to.
SPF Record Checker
Ten lookups and it stops working
Check an SPF record against RFC 7208: the ten DNS lookup limit that silently disables it, mechanisms after all, and the qualifiers that authorise everyone.
DMARC Record Checker
p=none protects nothing
Check a DMARC record against RFC 7489: whether the policy enforces anything, whether reports go anywhere, and whether pct is letting spoofed mail through.
DKIM Record Checker
An empty p is a revocation
Check a DKIM key record: the RSA key size read out of the key itself, the empty p that revokes it, and the t=y that asks verifiers to ignore failures.
DNS Record Checker
Legal syntax, wrong answer
Check a zone file or dig output for the mistakes that parse: a missing trailing dot, a CNAME sharing a name, an MX pointing at an alias, two SPF records.