[Exim] address verification for remote host

Hello,

One of our router directives sends incoming mail to a non-Exim host for
delivery into the end users' mailboxes. We've been having a lot of
problems lately with dictionary attacks and, if possible, I would like
our Exim mail-hub to be able to send the 550 rejection rather than
deliver the message on to the remote host. It doesn't appear this is a
function of the receiver_verify in global configuration nor the
verify_recipient directive in the router. Or if it is I've not set
something up properly.

Is it possible for Exim to query the remote host for an existing
account before the router sends it to the host, and if so which would
be the best way to do so?

Thanks for your help,
Todd