Dovecot infrastructure

Hi, I would need some suggestion for a dovecot based mail infrastructure. Actually, only one server (with dovecot sendmail amavis spamassassin clamav etc etc) is no longer enouth, so I thought I would put on a more complex infrastructure on different servers.

But I do not know a few things. I was thinking about partitioning users on 3-4 servers with dovecot (imap / pop3) and dovecot-lda (no networked filesystem between them) and then configuring 1-2 servers with dovecot director and 1-2 servers for SMTP with postfix. It is not clear how the director will map user -> server to be used and, in a user virtualization context (maybe on MySQL) what is the owner / permissions I should give to home folders where I will store mail

Thank you. Simone.