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

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.