Backup Relay: verify_recipient and differing transport_maps

[crayf...@gmail.com]

I am setting up a backup relay - and want to use the verify_recipient
feature (with a mysql query) to query the *final* destination of the
email for domains.

However - I don't want to send it there - I just want the email to be
queued to the normal MX destinations; and *not * use the transport.

The reason is that the primary MX is a filtering system which
ultimately delivers on to final destination using transport_maps; it
currently uses the verify_recipient feature, but of course it is going
to deliver mail there once filtered anyway.

I want the backup relay to perform verification but not attempt to
deliver to the final destination.

Alternatively I suppose I could use the content_filter on the backup
relay to insert the email into the primary -- but how does the queue
and retrying of mail destined for content_filter compare with handling
of relayed mail?

Ideas?

-Craig