huge email system

[David]

Hello -

We need to build a stable, redundant, and speedy email system that will last for a few years.  We need to handle about 500,000 emails per day.  We have about 30,000 users, so we need a lot of storage.

Our current plan was to implement the following.
2 SMTP only servers.
3 NFS servers with RAID and SCSI
2 POP3 servers.


But that leads us to questions such as -
 - what would be the best way to authenticate?
 - would the NFS servers need gig nic's? or dual bonded 100Mbit cards?
 - what smtp server and what pop3 server to use (we want to use Maildir)
 - what raid level?

Any suggestions on how to implement an email system to handle the load stated above would be greatly appreciated.

Thanks
David