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Every other page here answers one question well. This one asks four at once, and reports what their answers say about each other. That is the part you cannot see from inside any single check: a strict SPF record is no comfort if there is no DMARC policy to enforce anything, and a DMARC record set to "reject" protects nothing if SPF and DKIM never identify your own mail correctly in the first place.

What it looks at

SPF for which servers may send, and how strictly anything else is refused. DKIM for whether your mail is cryptographically signed. DMARC for what a receiving server should do when neither matches. MX for whether mail can arrive at all. And the addresses your SPF record names as senders, against the Spamhaus blocklist.

Where the interesting findings come from

Two records rarely disagree loudly. They disagree quietly, and only when you read them side by side. A subdomain that has no SPF record of its own while its parent's record names it as a sender is authorised to send as the parent, yet mail carrying the subdomain's own name is completely unprotected, because SPF does not inherit. Incoming mail arriving at one provider while SPF authorises a different one is often perfectly deliberate, and just as often an SPF record that never caught up with a migration.

What it does not do

Everything here is a DNS lookup. Nothing is sent, no mail server is connected to, and no message is delivered or inspected, so this cannot tell you whether your mail actually arrives. It also stops where guessing would start: DKIM selectors are derived from what your SPF record says about who sends your mail, rather than swept blindly, and where that yields nothing the check says so instead of implying there is no DKIM.

A note on timing

Nothing here is cached, so every run is a fresh lookup. A resolver still honours each record's own TTL, though, so a change published minutes ago may not be visible here yet. If you are actively editing records, temporarily lowering their TTL makes the feedback loop far shorter.