[psim1]

    $ dig example.com. mx +short
    0 .

It has an interesting MX record. I wonder what this does? Specifically, what should a mail agent do when the MX record points to "." ?

[oftenwrong]

That's a "null MX", indicating that the domain does not accept mail.

https://datatracker.ietf.org/doc/html/rfc7505

[psim1]

Thanks! That RFC is new to me.