I feel that I have a little different scenario, but equally demanding.
I haven’t see and tried the NS mail option; so I can compare with our current mail server, it use Citadel (I like it) in debian w/2 SATA drives (software raid). Is really simple to manage and just works. But I make a little mistake: I leave it un managed for a few months and the database grow so large that I can’t really do fast backups (it takes almost 2 hours of downtime). This version can’t recover free space from deleted message, but the new version will have that process.
Mostly we use POP user accounts, so I can keep and eyes in the space used on the mail server. A few are using IMAP; but I see some users messing up their IMAP folders.
Later I’ll try to find and read what email solution is used in NS, how the mails are stored, and what anti-spam/virus capabilities have. Including the administration tasks.