[acatton]

Not GP, but I think what they meant is having a MX record with a higher priority pointing to an unroutable IP

    blackhole  IN A 240.0.0.1

    @ MX 10 blackhole.example.com
    @ MX 20 mail.example.com

[jhugo]

Yup, exactly, for the reasons described in the sibling comment to yours.

Doing it this way doesn't even delay mail much most of the time; many legitimate MTAs connect immediately to the priority-20 MX after failing to connect to the priority-10 one.