Sender Policy Framework

What about the "implicit MX" rule?

When a domain has no MX records, SMTP (RFC 2821, section 5) assumes that an A record will suffice.

SPF can be similarly intuited. Some SPF libraries provide a "best guess" method, which pretends that domains without an SPF policy have a policy of "a/24 mx/24 ptr" defined. Even in the absence of SPF data, we can suggest that a transaction is legitimate (although we can't suggest that it is not legitimate, only that we don't know). And finding legitimate transactions helps other anti-abuse techniques reduce false positives.

Note: The "implicit MX" rule defined by RFC 2821 is considered generally problematic by many. For example, consider a mail server wanting to validate the mere "existence" of a domain used in an e-mail address. Assuming the "implicit MX" rule applies, any domain name that has even just an A record (i.e. every host name!) has to be considered a valid e-mail domain, whereas probably less than 0.01% of those domain names are actually used as a sender address in e-mail.