In article <CAKaVjbP2KyB5FGtzAqYjnaagg4P7+E+32d+2HFZnLTVUz7kphw@mail.gmail.com> you write:
1. Is there anyone actively using this Null MX? If so, may I please see that actual record line (in BIND zone file format) just to satisfy myself that I wrote mine correctly?
Yes. services.net. 3600 IN MX 0 .
2. Which one makes more sense from the practical point-of-view: having a Null MX Record for the no-mail domain, or having no MX record at all?
Null MX of course. See section 4 of RFC 7505. Large mail systems like gmail and recent versions of mail servers like Postfix all recognize a null MX. I suppose there are some dusty old mail systems that do something odd with it and throw an error message but so what? You don't want them to send you mail in the first place. R's, John