Checks whether a domain can receive and send email cleanly: mail servers, encryption, SPF, DKIM, DMARC and blocklists. No email is sent.
The check reads public DNS records and briefly connects to the domain's mail servers. Without a selector the usual names are tried (mailcow uses dkim).
The domain's public DNS records and what its mail servers show when a connection is opened: greeting, encryption and certificate. Then we close the connection. No email is sent and no mailbox is contacted.
SPF defines which servers may send in the domain's name. DKIM adds a signature to every message that recipients can verify. DMARC tells recipients what to do with mail that fails both. Together they stop others from forging your sender address.
Whether an individual message arrives also depends on its content and on the sender's reputation at Gmail, Outlook and others. Their internal ratings are not visible from outside. The check shows whether the technical foundation is sound.