[toast0]

> Why didn’t the DMARC spec say that a domain is “not present” if it lacks MX records?

That doesn't match the SMTP spec, RFC 5321 says

> If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host.

https://www.rfc-editor.org/rfc/rfc5321.html#section-5

[ZWoz]

DMARC isn't part of SMTP.

Now, steelman argument is that DMARC is used together with SMTP, so it should be somewhat compatible to work in practice. But in this case there isn't fundamental conflict breaking SMTP.

[em-bee]

DMARC demands extra DNS entries that SMTP doesn't need, so there is no reason why DMARC could not also demand an explicit MX record even if plain SMPT doesn't