[emmelaich]

> The domain has a null MX record (indicating it doesn't accept email)

Not quite true, SMTP will use the A record if there is no MX.

[dpifke]

In this case, "null MX record" means MX exists, but does not specify a valid server:

   $ host -t mx example.com
   example.com mail is handled by 0 .

Senders should not fall back on the A record in this case.