easiest way is to just download the file, since you really don't need to
have the whole repo for the one file, unless there are unfilled
dependencies..
quick way: rpm -ivh
http://centos.karan.org/el4/extras/stable/i386/RPMS/grepmail-5.3032-2.el4.kb.noarch.rpm
(found by your google search)
or wget
http://centos.karan.org/el4/extras/stable/i386/RPMS/grepmail-5.3032-2.el4.kb.noarch.rpm
then rpm -ivh grepmail...
also,
dag has a great FAQ section -
http://dag.wieers.com/home-made/apt/FAQ.php#B4 on how to add
repositories for yum.
hth,
cameron