[Dovecot] SASL auth and proxy

[mailing at securitylabs.it]

  Hello, I'm finding a way to have a pool of smtp-auth servers in front 
of my actuals servers to permit relay to our customers. Now users sends 
emails connecting to the right IP via mail.domain.tld, with 
authentication. Every server has it's own user database in mysql.

I'd like to have a pool of server (with round robin, ideally with 
postfix + sasl + dovecot) that authenticate the users with the actuals 
servers and then pointing mail.domain.tld to this pool and not to one 
server.

My trouble is how to authenticate users.

Can I use:

http://wiki.dovecot.org/Authentication/MultipleDatabases

to tell Dovecot to authenticate with every MySQL database on the servers 
"behind"?

If yes, is there a better way to do this considering I'm already using 
some proxy machines (dovecot) for pop3/imap and so I already have a db 
with domain.tld -> IP of the server? I also have some machine without a 
MySQL DB for users (vpopmail + cdb), so the option with 
MultipleDatabases will be good only for some machines.

Something similar to the proxy mechanism that I already use for 
pop3/imsp will be good: the proxy connect to the machine behind that do 
the authentication, I only need a table with domain -> server IP (that I 
already have).

Thanks for any advice.