Interesting! I did not know this For those wondering, this is actually specified in rfc5321 section 5: If an empty list of MXs is returned, the address is

Interesting! I did not know this For those wondering, this is actually specified in rfc5321 section 5: 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://datatracker.ietf.org/doc/html/rfc5321#section-5 https://datatracker.ietf.org/doc/html/rfc5321#section-5