We frequently receive emails that we're not prepared to act upon immediately. These emails sit in our inboxes, taking up space and causing a cluttered appearance. Of course, these "pending" emails could be tagged as "Pending" and moved into another folder, but we all know what happens next...in our busy lives, we forget to check the "pending" folders on a regular basis and the emails are fogotten about until weeks or months later, by which time it's too late to act on them. It's happened to me plenty of times and I'm sure it's happened to many other people as well.
If Zimbra's mission is to "fix email", may I suggest that one of the most important "fixes" they could offer would be to provide an automated scheduling mechanism that would help users clean out their inboxes, without worrying about emails becoming lost or forgotten. I would propose a right-click context menu action called "Defer". When you select this action, a submenu would appear with options like "Tomorrow, Two Days, Three Days, Next Week, Next Month, Specific Date". If you choose the "Specific Date" option, then a popup calendar would appear and could select the exact date that you want to defer the email to.
Once an email is deferred, Zimbra would automatically move it to a "Deferred" folder and/or apply a "Deferred" tag. The important point is that the email would be out of your inbox, allowing you to focus on the tasks at hand. However, on the scheduled date, the email would reappear in your inbox to remind you that it's time to act on it.