Dovecot as Proxy for __MORE__ Exchange-Servers

[Luca Bertoncello]

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 at lucabert.de)