Domain Checker

One combined view of SPF, DMARC, MX, MTA-STS and TLS-RPT for a domain, plus DKIM if you know the selector. There's no way to discover a DKIM selector for an arbitrary domain, so that part only runs if you supply one — this doesn't guess.

What this checks

SPF — record and DNS lookup count. Full detail at the SPF Check.
DMARC — record and policy. Full detail at the DMARC Inspector.
DKIM — only when a selector is supplied. Full detail at the DKIM Inspector.
MX records, including a null MX (RFC 7505) if this domain deliberately doesn't accept mail.
MTA-STS — whether a policy is published, whether the policy file is actually retrievable, and its mode. Full detail, including whether the policy still covers every MX host, at the MTA-STS Checker.
TLS-RPT — whether SMTP TLS failure reporting is configured.