THere's a problem with your download, not the package.
It's possible that your browser is trying to extract the file on download, or that you've got a proxy server (are you at work?) that's stripping out the contents.
Try fetching it with wget (on your linux box):
wget
http://downloads.zimbra.com/3.0_M1/z....M1_21.FC3.tgz
which should get around any browser stuff that may be happening.