The mail server for 500 users I mentioned recently is ready to rumble. It runs on CentOS 4.2 and uses DRBD to replicate a 500 Gig file system from a master to a hot-standby machine. These two are crosslinked with Gigabit Ethernet. heartbeat uses that same link as well as a dedicated serial link to ensure timely failover in case the primary system goes down. So far, I am very pleased with both the performance and stability of DRBD indeed.
The wonderful Exim is used as an MTA, and Dovecot provides IMAP services to the clients (I do wish Dovecot would support Maildir quotas soon). For web-based IMAP access, SquirrelMail does the job very nicely, together with some plugins.
The whole system is set up like a kind of mail toaster, meaning that no "real " users live on the system: all is virtual. OpenLDAP handles storage for all objects required for that virtualization (e-mail domains, user objects, preferences, etc.), and it does so very well and fast.
As I said: we are ready to rumble!