Pro move: type dmarc:yourdomain.com or blacklist:1.2.3.4 straight into the box — it switches tools for you.
About this tool. Connects to a mail server, reads its greeting banner, sends EHLO and lists what the server supports — including STARTTLS. our edge (like most clouds) can't open port 25, so we test the submission ports 587 and 465, which is what your own devices use to send mail.