I posted this request (Headers only ZCO?) a while ago and got what we asked for. However, while we were preparing to upgrade and looking for comments from other users in the forums, we found this (Zimbra 6 download headers only, question/issue) along with the description of headers only functionality in the documentation.
Now I'm again asking for people to vote for another enhancement request (Bug 38520 - headers only mode: download message when read). Someone might want to make two enhancement requests out of that particular bug. We feel that with either enhancement, on-demand downloading or a download size threshold, we could get users to understand and make good use of the new feature.
(There is a duplicate post in Administrators forum)