If your mail directory is mounted from another machine, your machine
  does not have root access to that directory, and the other machine has
  BSD-style "restricted chown" (the restricted_chown kernel variable is
  turned on or it's not an SGI) /bin/mail will change mail file
  ownership when delivering local mail. Without unrestricted chown *or*
  root access, /bin/mail is unable to give a mail file back to its owner
  after delivering mail. You can fix the problem by turning off re-
  stricted chown on the other machine (if it's an SGI) or exporting the
  mail directory with root access for your machine. This problem was to
  have been fixed in IRIX 5.2, but apparently is still present in 5.3.