system-wide mailbox on aix

[W. W. Looi]

Does anyone know why there are two system-wide mailboxes on AIX?
They locate in /usr/spool/mail/$USER and /usr/mail/$USER
Why is it neccessary to have two mailboxes? If /usr/mail/$USER is 
just a dupe of /usr/spool/mail/$USER, can I remove it altogther
to conserve disk space? Will removing it lead to any side effect?

Any insight will be appreciated.

-looi