[Exim] receiver verify from a mail hub

Hi all,

We use Exim on a mail hub machine which has a hub_route that accepts
all mail for all our domains and then routes it via remote_smtp
transport to whatever machine holds the mailboxes for a particular
domain.

We've recently set up with Postini for site-wide service. Their system
creates, or doesn't create, an account based on an smtp query to the
machine that is used to receive incoming mail, which in our case is the
mail hub. Basically if an email to "anyone@???" passes
through their system they send a quick little smtp telnet to the
specified machine, and if it doesn't return a 550 error the account is
created and the mail delivered. If it does get a 550 error the account
is not created and the mail is also delivered to just bounce as normal.

Because of this hub -> forwarder arrangement, however, all
verifications are coming back positive. I really don't want to have
them deliver to any other machine than the hub.

Is there any simple way for me to set up Exim to send a 550 error for
invalid addresses directly from the mail hub?

Hope I've explained this properly....

Thanks for your ideas,

Best,
Todd