Name of queue dirs split accross subdirs
hash_queue_names
This parameter specifies the names of queue directories that are split across
multiple subdirectory levels. Hashing MUST NOT be used with a world-writable
maildrop directory. Hashing MUST be used for the defer logfile directory, or
mail system performance will suffer.
Unfortunately, hashing the incoming or deferred queue can actually
slow the mail system down (mailq with an empty queue can take
several seconds) so it should be done only in case of emergency.