There are four directories on a mail server, depending upon the
components being used on a given server, that have more I/O activity
than others. These directories are:

msg-<instance>/imta/queue/
msg-<instance>/logs/
msg-<instance>/store/mboxlist/
msg-<instance>/store/partition/primary/

Those directories contain files that are accessed very frequently. Having those files on the same disk or set of disks can degrade performance. Ideally each of those directories will have its own disk or even better, a set of disk spindles (RAID).