This specific server is a dual core 2.8ghz xeon with hyperthreading running on FreeBSD 6.2-STABLE. We have over 1 million mailboxes, with about 75,000 daily active users. At peak maybe 20,000 concurrent, in a mix of webmail and direct imap. (no POP, thats handled by different software).
The backend is a NetAPP 6070 with about 170 harddisks. All mail and control files are on this netapp, but all indexes are on local disks, and we try and make sure customers end up on the same imap server every time. As soon as all servers are 1.1 I'll start experimenting with NFS indexes.
Just in case I understand you wrong: You're serving 20k concurrent users with 1 (one) server?
What mailbox backend are you using? maildir?
Actually..it's 2 NetApp 6070s. But those are not just simple servers. They are very expensive, dedicated NFS boxes each taking up a full rack doing multiple terrabytes each, connected with multiple gbit links.
It's 99.99% maildir, with a very small hidden mbox legacy namespace.
Cor