Exim + Spamassassin + TMDA

Hi,

I would like to use Exim + Spamassassin + TMDA to fight spam.
(I know about the discussion about TMDA, but I need it for some reasons).

The procedure should be:

Mails which are not tagged by Spamassassin are delivered directly.
Mails which are tagged by Spamassassin as possible spam should be handed
over to TMDA and first be frozen. After confirmation due to
challenge/response the message is unfrozen and finally delivered.

Whether a message should be tagged as spam can be defined within the
spamassassin configuration itself - as usual.

I think I would freeze only mails tagged as spam between some score x and y
and move to the trash mails with spam score higher than y directly.

Can anyone explain how to do this maybe ?

Thanks.

Regards.

P.S.: I am using Exim with virtual domains support, but I am not sure if
this makes a difference at all.