CentOS Errata and Security Advisory 2006:0283
https://rhn.redhat.com/errata/RHSA-2006-0283.html
The following updated files have been uploaded and are currently syncing to the mirrors:
s390: updates/s390/RPMS/squirrelmail-1.4.6-5.el4.centos4.noarch.rpm
s390x: updates/s390x/RPMS/squirrelmail-1.4.6-5.el4.centos4.noarch.rpm