https://rhn.redhat.com/errata/RHSA-2005-136.html refers

updated files are :-

updates/x86_64/RPMS/mailman-2.1.5-24.rhel3.x86_64.rpm

updates/x86_64/SRPMS/mailman-2.1.5-24.rhel3.src.rpm

To update CentOS 3 to this new version run 'yum update mailman'

CentOS 3.3 shipped with an earlier version of mailman - mailman-2.0.13-5
- in the extras repo.

Details of the mailman upgrade procedure may be found at
http://www.centos.org/mailman

The 3.3 x86_64 updates repository already contained a
mailman-2.1.5-23, so the upgrade was also provided for 3.3 and 3.4.

Consider upgrading to CentOS 3.4
(http://www.centos.org/modules/news/article.php?storyid=34)