Mail storage in local disk

Filesystems

See MailboxFormat/Maildir for Maildir-specific filesystem optimizations
Dovecot doesn't rely on atime updates, so you can mount the filesystem with noatime

Index files

Keeping index files on a different disk than the mail spool gives you better performance. The indexes have a lot of write activity so it is recommended to use RAID-10 instead of RAID-5 for them.

Fsyncing

By default Dovecot calls fsync() and fdatasync() whenever it's useful to prevent potential data loss. The main reason for this is so that Dovecot won't lie that the message was saved to the disk, if in fact a power failure a second later would lose the message. With IMAP clients this is perhaps a less serious problem, because the lost message was most likely either a mail in Draft mailbox or a message in "Sent Messages" mailbox. In any case a message that the user had already seen. However if LDA loses a message, the user never even knew that the message existed, unless the sender decides to resend it.

Since power failures and kernel panics are quite rare, many people are tempted to disable fsyncing because it may increase the performance quite a lot. Dovecot allows this by setting mail_fsync=never. However you should consider leaving it at "no" for LDA and LMTP. You can do this with:

# Default to no fsyncing
mail_fsync = never

protocol lda {
  # Enable fsyncing for LDA
  mail_fsync = optimized
}
protocol lmtp {
  # Enable fsyncing for LMTP
  mail_fsync = optimized
}