Dovecot as Proxy for __MORE__ Exchange-Servers

Hi list!

I'm trying to configure Dovecot 2.2.18 to act as proxy. Dovecot has to authentificate the users against an LDAP-Server (AD)
and then act as IMAP-Proxy to an Exchange Server.

Now the problem is, that we have TWO Exchange servers (and two ADs, too). How can I say to Dovecot that it has to search for the user in the
first AD (and if found proxy to the first Exchange server) and if the
user was not found, to search in the second AD (and if found proxy to
the second Exchange server)?

Thanks a lot Luca Bertoncello (lucabert@lucabert.de)