[Dovecot] Architecture for large Dovecot cluster

[Murray Trainer]

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