/export/mailboxes/j/o/h/n/johndoe.mbox
In the past I've actually found that reversing the letters gives much better randomosity around the directory structure so, johndoe@clown.org would end up in e/o/d/n/johndoe and you don't take much of a hit for this.
very costly, though, because of all the recursive directories. Also, you're going to end up with some directories very imbalanced, since there are more frequently occurring names.
It also makes backups a nightmare. In that case, you'll have to shutdown the entire mail system before you can backup or you'll have a database image which won't represent the actual data you have on your NAS.
In a high performance/availability system typical tape/spool based backups are problematic - with netapp you have a number of options to handle this [snap mirror etc]. It really depends on your turn over of data which for mail is usually pretty high. [oh and IBM disks tend to make a huge difference :-)]. Ofcourse spool type backups are fine for the OS and configurations. Regards, Neil.