A secondary MX record doesn't solve the problem of "accept email and silently route to /dev/null".
Aside telling the sender or receiver, there's no good way to know. I guess you could send a test email every hour.... But this is an evil failure mode. This isn't an accident.
Aside telling the sender or receiver, there's no good way to know. I guess you could send a test email every hour.... But this is an evil failure mode. This isn't an accident.