/var/qmail

This is the qmail home directory. It must be a local directory, not
shared among machines. This is where qmail queues all mail messages. 

The queue (except for bounce message contents) is crashproof, if the
filesystem guarantees that single-byte writes are atomic and that
directory operations are synchronous. These guarantees are provided by
fixed-block filesystems such as UFS and by journaling filesystems. Under
Linux, make sure that all mail-handling filesystems are mounted with
synchronous metadata.