[exim] catch-almost-all forwarding

One association had their mail hosted by a friendly ISP.
All the contacts where made with one person of the staff of the ISP that
cared the updating of userlists and distribution lists.
Now this person is in a sabbatical year traveling in remote plagues of the
world and sometimes answering the email after more than one month.
The update of most lists is urgent so we decided to put all in one of
the association's server, given also that the ISP announced that free
hosting of mail service would end on june.
So we faced the problem that we know the content of most lists, but we do
not know the content of one list and more important we do not know the
list of addresses xxx@??? that are set, we know a few ones but
could be more .
We tried to ask the isp so can give us the list but they said that since
all the arrangement were made by that person, before passing the
information they must have approval by that person, or at least timeout
for his reply. So probably until end of april we will not have any
support.
So my idea was to set the new server, moving to it the MX, setting all the
lists and user we know, and for unknown adrresses forward it to the
current server.
So we could know which are the active address, and being able to contact
the owner to complete migration (if an address in two months does not
receive a messge we can assume it is dead), at the same time if the user
is active the service is not disrupted.
(please do not ask why there are some user on the domain and the owner of
the domain do not have the list ...).
Of course i cannot make a normal catch-all that route "noirmally" since
the mx for the domain is the server that is forwarding, so it must forward
the messge without canging it, but forwarding to a different machine that
is no longe the one on mx record ...
(actually: even email generated locally and sent to "unknown" addresses
must be forwarded that way)
suggestion on the safest way higly welcome !

--
Leonardo Boselli
Dipartimento ingegneria Civile e Ambientale UNIFI
tel +39 0552758808