"Most certainly" doesn't mean "always". One example: mz.ru is undeliverable
but perfectly valid domain of mailing list server mailzone.ru. These gyus
There's nothing in mailzone.ru's DNS that refers in any way to mz.ru
intentionally or erroneously configured their domain as undeliverable, but
If @mz.ru is undeliverable - and it is - then using @mz.ru to send mail to
servers that reject mail from undeliverable domains is not going to work.
That's what BOFHCHECKDNS does. If you want to accept mail from
undeliverable domains, turn it off.