[kirab]

No offense but I think you got the meaning of the record wrong:

https://community.cloudflare.com/t/keep-null-mx-in-addition-...

> Null MX does nothing to prevent spoofing. It is just to signal to mail senders that your domain does not receive email.

And I also found multiple other sources that specify that the null record only announces that the domain does not receive email. After some googling I didn't find a single source that shares your explanation.