What the MX lookup checks
The tool reads the MX records of the domain and lists each mail host with its preference value. Where the hostnames match a well-known provider pattern it names the likely provider. It then fetches the SPF policy from the TXT records of the domain and the DMARC policy from _dmarc under the domain, parses both and reports findings.
Everything here is read from public DNS. The tool does not connect to your mail server, send test messages or look inside any mailbox.