#
20 Servers: SuperMicro Dual Intel(R) Xeon(TM) CPU 3.20GHz Raid1
#
OS: FreeBSD 4.10
#
RAM: 4 Gb
#
Users: 500,000+, 5000+ simultaneously active.
#
Authentication: passwd/pam
#
Storage: Several terabytes served by NetApps.
#
Users Maildir Size: 500MB per user.
http://wiki.dovecot.org/DovecotServerInstallations/FreeBSD/4.10/500000Users