A null MX is a single MX record with priority 0 and an empty host (.), defined in RFC 7505:
example.com. MX 0 .
It tells every sender: this domain does not accept email. Domains used only for websites, or reserved domains like example.com, publish it so mail fails fast instead of queueing.
Any address at a null-MX domain is undeliverable. Veqen reports it with the domain_refuses_mail reason.