We'll see what others say, but here's my opinion.
The big bottleneck with a mail server is going to be your bandwidth. You can optimize your array for I/O times that resemble lightning, but if you only have a 1.5 or 3 megabit Internet service, it's not going to make a lot of difference. As each mail comes in, your server will say, "bzzt!!!" and process it in a couple of microseconds, but then it bottlenecks waiting for the next data to come in through that relatively-limited (by comparison) data pipe.
If it was me, I'd go with the default (which is 64KByte, I believe). A mail server has a lot of variables -- people receive new mail, delete old mail, one guy gets huge attachments while the next only gets an occasional, "this is mom, why don't you write?!?" In other words, don't view this as a typical database server. It isn't. Scalix has a database, but it's basically an *index* of pointers into the mail and/or a list of users. You're not going to have millions of fixed-size data records to deal with, you're going to have a mail store that grows and shrinks with usage. See what I'm saying?
But still, I say it's a smart move on the Raid, by the way. Don't know if I'd go all with Raid 10; ee have just a basic Raid 1 with two 500 Gig disks. We deliberately take a performance hit on disk I/O to do 100% mirroring. (See what I said above: your bottleneck isn't going to be disk I/O, it's going to be your Internet connection.)
But it's still worth it. That Raid has saved our bacon TWICE in the past month and a half -- we lost one drive, kept running on the second, replaced the first drive. Then the second one died, we kept running on the (new) first until we replaced the second one!
Proves what Carnegie Mellon and Google have found out in separate studies, by the way ... assuming all of the same disks, same type and same approximate age, if you lose one in a RAID array, you are almost certain to lose another one within a few weeks. I didn't believe it either until it happened to me. (Twice.)