The mail spool must be accessible through
/var/mail and the mail spool files must take the
form <username>.
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User mailbox files in this location must be stored in the standard UNIX mailbox format.
The logical location for this directory was changed from
/var/spool/mail in order to bring FHS in-line
with nearly every UNIX distribution. This change is important for
inter-operability since a single /var/mail is
often shared between multiple hosts and multiple UNIX distribution
(despite NFS locking issues).
It is important to note that there is no requirement to
physically move the mail spool to this location. However, programs
and header files must be changed to use
/var/mail.