edward the puppy eater (part 3)

so i was wondering, is it possible to have postfix check that the primary mx is not up before receiving mail for a backup domain? that is, when it receives an 'rcpt to' that it has validated as a domain it's relaying for, it will make an smtp connection to the primary mx of the domain (using whatever resolution it uses when forwarding relayed mail) and if it succeeds, it will return a temporary 400 code in response to the 'rcpt to' telling it to send to the primary mx instead.

i searched around and couldn't find anything that implemented this, but i'd imagine someone's wanted to do this by now.