It checks if the email domain has the following records set up.
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MX - Mail Exchange: Checks if the Mail Exchange for the domain has been set up in the DNS.
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SPF - Sender Policy Framework: Checks if the domain has set up SPF records with the authorized IPs that are allowed to send mail from this domain.
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DMARC - Domain-based Message Authentication Recporting and Conformance: Checks if the domain has set up DMARC records with the list of action what should be done if the emailing server fails to pass the SPF and DKIM check.
Verify DKIM (DomainKey Identified Mail) for the tested domain.
Reference: