[Dovecot] Architecture for large Dovecot cluster

Hi All,

I am trying to determine whether a mail server cluster based on Dovecot will be capable of supporting 500,000+ mailboxes with about 50,000 IMAP and 5000 active POP3 connections. I have looked at the Dovecot clustering suggestions here:

http://blog.dovecot.org/2012/02/dovecot-clustering-with-dsync-based.html

and some other Dovecot mailing list threads but I am not sure how many users such a setup will handle. I have a concern about the I/O performance of NFS in the suggested architecture above. One possible option available to us is to split up the mailboxes over multiple clusters with subsets of domains. Is there anyone out there currently running this many users on a Dovecot based mail cluster? Some suggestions or advice on the best way to go would be greatly appreciated.

Thanks

Murray