RFC7505 Means Yes, Your Domain Can Refuse to Handle Mail

I was curious why a "null MX" is even needed. If there are no MX rrs it would mean that this domain doesn't handle mail, right? It's actually because of the "implicit MX" rule defined in rfc5321[0]. If there are no MX rrs, the SMTP client will still try to use the A or AAAA records to see if there are any SMTP listeners. Adding a null MX "0 ." will cause the delivery to fail immediately.

[0] https://tools.ietf.org/html/rfc5321#section-5

gov.uk advice is to set up a null dkim record if you don't send email, so you can't be spoofed.

https://news.ycombinator.com/item?id=25210245