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.
it is already enormous overshoot in hardware specs. And i do not really catch why you have "4 in parallel" servers.
And finally i cannot understand this dividing of servers just to merging it back using VMWare.
Finally i would recommend to get rid of RAID6. It's terribly slow on writes and writes are common on mail server.
Buy cheapest but largest SATA drive and use RAID1 (or RAID1+0) setup.