You may want to try to do what we do for archival purposes.
We have another computer running OSE, when an account is closed, I export the account as a tar.gz file, and then import it to the OSE box before removing the account off of our NE box.
You could pick the largest accounts that you have for archiving, export them. Move them to the OSE box and then from the Zimbra command line, start doing mass purges on specific folders.
Another option would be, if they're using Outlook, to use the built-in archiving facility of Outlook and archive to a local folder on their machine.
Ben Franklin quote:
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety."