5.7.2. NFS and Sendmail

Never put the mail spool directory, /var/spool/mail/, on an NFS shared volume.

Because NFSv2 and NFSv3 do not maintain control over user and group IDs, two or more users can have the same UID, and receive and read each other's mail. With NFSv4 using Kerberos, this is not the case, since the SECRPC_GSS kernel module does not utilize UID-based authentication.