I followed the link on the Open Source Edition Downloads page and downloaded Zimbra 4.5.1 for Red Hat Enterprise Linux 4 (filename zcs-4.5.1_GA_660.RHEL4.tgz) from sourceforge. However, I installed it and it appears to be the Network Edition version that requires a license. There is only the admin account and I cannot create any others.
Is this a mistake? If not, where can I access the open source edition of Zimbra 4.5.1 for RHEL4?