[Exim] verify recipients when being a mailhub

I am using exim as a central mailhub for two domains; it forwards the
incoming mail to the respective domains via the domainlist router. Works
like a charm.
Now we have some unknown (i.e. former) users that receive some spam; of
course, exim forwards it to ex-users correct mail server which generates a
'no such user here' message which then arrives back at exim for delivery.
Sure enough the sender domain won't answer (silliest reply I've seen so
far is "Too busy to handle your request") so the msg is frozen.

How can I implement a receiver_verify thingie at SMTP time (when exim
receives original msg from sender) that will check for invalid users (not
too many, so it's easier to implement a bad_user_list instead of a
valid_user_list).?

In the meantime I just let frozen messages die after 24 hours.

--Ben (bhoc at surfeu in ch)