[Dovecot] Dovecot Proxy

Hello

I have installed Dovecot as a POP3 server, IMAP on my internal
network, the authentication against Windows Active Directory.

I would like to know how to setup a second server dovecot as my proxy
server for external users outside the network, in another segment of
different IP addresses, or at least define which mailboxes will be
stored on another server to be accessible to both servers.

I have reviewed the document on the wiki site, but I said that is only
possible with mysql authentication, but as I said the road where the
messages are stored, it is possible for NFS?

Thanks

Michel