Server: Dell PowerEdge 1750, Dual Xeon 3GHz HT, 4GB RAM, 3x 74GB 15k SCSI, RAID level-5 PERC 4Di 128MB BBU.
Total mailboxs size: 100G, we have quota 
Software: Latest Dovecot, CentOS 5.x, Mysql 5.0 user db.
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:
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