Clustering mail servers - Cyrus or Courier ?

[Przemyslaw Wegrzyn <czajnik@tower.t16.ds.pwr.wroc.pl>]

I'm looking for good solution for big, scalabale mailserver installation,
for 500,000 accounts and more... 

As for now I think:
 - sharng NFS-mounted mail storage is not so good idea.
 - scaling can be done by partitioning mail storage between servers (with 
a little less avialabilty risk).

However, AFAIK it can be done only with Cyrus with its IMAP Aggregator, or
with qmail-ldap + Courier-IMAP...

I started doing such installation with Courier-IMAP + qmail-ldap, and it's
nice, expect one thing - quota support. I need _reliable_ quota system..

Tis forced me to look at Cyrus, with its good quoa support, however, I
have no experience with Cyrus.

Would you give some advices ? Any comparision benchmarks ?

Greetings
-=Czaj-nick=-