Perhaps for one of the following reasons:

  - IRIX 5.2 NFS has a bug which prevents writing to a zero-length file
    if the writing process has group write permission but not group read
    (or user write) permission. IRIX 5.2 /bin/mail likes to set the
    permissions of /var/mail/<user> to mode 620, exactly what is needed
    to exercise the NFS bug. There is no patch for these bugs, but both
    are fixed in IRIX 5.3. Meanwhile, you can work around the problem by
    chmod'ing your mail files to 660 and setting the appropriate option
    in your mail program to make sure that empty mail files are not
    removed.

  - lockd must be running on the NFS server to allow programs on the
    client to lock mail files. If it is not, Zmail/MediaMail will
    complain that "file cannot be locked". One could tell MediaMail to
    not request a lock with "unset dot_lock", but that would risk mail
    file corruption.

  - IRIX 5.3 lockd is broken. Typical symptoms include "Warning: could
    not lock /var/mail/user after 100 trys." (from /usr/sbin/Mail) and
    the presence of user.lock and user.rolock files in /var/mail. Patch
    1128 fixes at least some of these problems.  If the client and
    server are both IRIX 5.3, you can also work around the problem by
    changing the mount type to NFS3. (See notes on NFS3 elsewhere in
    this FAQ.)

  See also "NFS Mounting Mail with IRIX 5.3 and IRIX 6.0.1" in the
  Sep/Oct 1995 Pipeline.