If you want a preview of the webpages when you mouseover the url you have several options/methods of enabeling:
su - zimbra
cd /opt/zimbra/zimlets/
1) zmzimletctl dumpConfigTemplate com_zimbra_url.zip
nano config_template.xml
edit:<property name="disablePreview">true</property>
change it to false, and run
zmzimlet configure config_template.xml
2) cp com_zimbra_url.zip somewhere
unzip com_zimbra_url.zip
nano config_template.xml > edit edit "disablePreview" to false
zip it back up and run
zmzimletctl deploy com_zimbra_ul.zip
(or 'install' instead of 'deploy')
If you have a Alexa / Amazon thumbnail account you can also enter that account information as well.
Go and mouse over a url, you should see a preview - if it's blank the first time it needs to generate the image - try again a little later.
Also make sure it's enabled on your COS/User (deploying only enables on the default COS).
If you want to force faster generation visit
websnaper.com and use the address bar given to make it garb one on the spot (select small 202x152) then go back to zimbra and try it.
If still nothing after lots of work, then try adding it to the allowed domains:<property name="allowedDomains">ast.amazonaws.com, *.websnapr.com</property>