`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.