Server: Dell PowerEdge 1750, Dual Xeon 3GHz HT, 4GB RAM, 3x 74GB 15k SCSI, RAID level-5 PERC 4Di 128MB BBU.
Since 2010/2011 year we are on Dell PE 2850, Dual Xeon 2.66GHz HT (lower clock due less watts), 4GB RAM, 6x 74GB 15k SCSI, RAID level-5 ...
Antispam and webmail are another boxes.
Total mailboxs size: 100G (2009/08), we have quota 
Since 2010/2011 300G and using multi-instance (8x).
Software: Latest Dovecot (1.x), CentOS 5.x, Mysql 5.1 (5.0 before) user db on Myisam, Postfix 2.7 (postmulti used).
Users: 10,000, POP, IMAP, SquirrelMail, SMTP-AUTH.
Mailboxformat: maildir
We used Courier-IMAP before, works quite well, but Dovecot have lower load and is more friendly/configurable IMHO.
Every */5 hours rsync to hotswap server (HA via heartbeat).
Sar stat (2009/08):
CPU %user %nice %system %iowait %steal %idle Average: all 2.81 0.56 1.66 1.65 0.00 93.32
free -m:
total used free shared buffers cached Mem: 3804 2903 900 0 357 1265 -/+ buffers/cache: 1280 2524 Swap: 4094 0 4094
Sar stat (2011/01):
CPU %user %nice %system %iowait %steal %idle Average: all 5.32 0.41 3.47 5.71 0.00 85.10
free -m:
total used free shared buffers cached Mem: 4046 3723 323 0 289 2385 -/+ buffers/cache: 1048 2997 Swap: 4000 25 3974
Since 2010/11 we had problems with limits, currently runs ~400 imap and ~400 imap-login processes
IMAP: Warning: Inotify instance limit for user exceeded, disabling. Increase /proc/sys/fs/inotify/max_user_instances (set to 128)
Warning: All login processes are in use. You may need to increase login_max_processes_count (default 128)
Feel free to contact me at fuxa at unihost.cz if you have any questions.