A patch for shadow-utils 20000902 causes the useradd command to create a mail spool files with read/write privileges of the new user's group (mode 660), which allows other users in the same group to read or modify the new user's incoming email.
http://www.redhat.com/support/errata/RHSA-2003-058.html
http://www.redhat.com/support/errata/RHSA-2003-057.html
http://www.mandrakesoft.com/security/advisories?name=MDKSA-2003:026