CentOS Errata and Security Advisory 2005:595
https://rhn.redhat.com/errata/RHSA-2005-595.html
The following updated files have been uploaded and are currently syncing to the mirrors:
s390: updates/s390/RPMS/squirrelmail-1.4.3a-10.EL3.centos.1.noarch.rpm
s390x: updates/s390x/RPMS/squirrelmail-1.4.3a-10.EL3.centos.1.noarch.rpm