[Dovecot] Hardware infrastructure for email system

[Michael Wessel]

Hi,

I'm currently (re-)planning my email setup and have been doing some 
research. I have done some searches and read several threads in the 
areas of my questions here. While there are some that come close I 
haven't yet been able to get all my questions answered.

I currently run a postfix, dovecot & roundcube setup and have about 2000 
active accounts. I have a separate SMTP server for outbound mail and 
auth is done against a separate LDAP server. In front of the POP/IMAP 
server I have another SMTP (4 in parallel actually) server that receives 
and filters inbound mail through a company specific, proprietary filter 
before the mail hits the POP/IMAP server. LDAP & SMTP servers are ESXi VMs.

So right now both dovecot and roundcube run on the same box which is a 
Dell PE2950 with dual quad-core Xeon, 16GB RAM and 6 1TB disks in RAID 
6, so only local storage using maildir.  So far it's been holding up 
fine, but it's beginning to show signs of overload now. I also expect an 
increase in users over the next few months up to somewhere between 10 - 
20,000 mail boxes. Hence the re-planning.

My first priority in redesigning my setup is reliability. I definitely 
need something fail-save and as close to always on as possible. Next is 
performance. And while the budget is of course limited for the moment 
I'm setting that aside and will worry about that when the time comes.

Now here is my question(s):

In order to support up to 20,000 mailboxes (distributed over several 
times-zones so they won't all be used at the same time) with a very 
reliable service with good performance, what do I actually need?

Do I need(ul) SAN or is it just a "would be nice to have"? If yes, why 
and what would be appropriate for my needs? Or will a setup with a few 
more servers like the ones I already have, using something like DRBD and 
distributing services (imap, http, spamd etc) onto different boxes do?

I know I have more reading to do on all the different options out there, 
but would like some input from people that have experience in this area 
so I can focus on the stuff that's right for my situation.

Michael