The default /etc/aliases lacks aliases for some Red Hat pseudo-users, such as adm and mail. The spool directory will silently fill with mail to these users. I recommend adding additional aliases that redirect these users' mail to root. Aliases to add include: adm lp sync shutdown halt mail news gopher ftp nobody postgres nobody's mail should probably dump into /dev/null.
Missed a few, here's some more: httpd xfs gdm squid