Every other check here reads what your DNS says. This one reads a message you actually sent, and tells you what a receiving server would make of it.
Send from the account you are testing, then reload this page. The address works once and is deleted in 59 minutes, along with the message.
Nothing has arrived yet. Send one and this page will update on its own — no need to reload, though reloading works too.
If it does not appear, your server may have refused to deliver to a new domain, which is itself worth knowing. The result will say so when it arrives.
A DNS checker tells you what your domain claims. It cannot tell you what your server did. The two come apart more often than people expect: the right key published while the server signs with a different one, a body rewritten in transit by a gateway or a mailing list, or SPF passing for an envelope address that has nothing to do with the name the reader sees.
That last one is the expensive one. A message can carry a valid DKIM signature and a clean SPF pass and still be rejected, because both belong to the sending platform rather than to the domain in your From header. Every record looks correct, every checker says green, and the mail bounces. This page names that mismatch directly.