How to: Mark All Messages unread in Zimbra Webmail
1- Login to your Zimbra webmail, click on Inbox on a folder to display your messages.
2- [left-click + shift] the checkbox at the top-left of the list of messages: ALL messages are selected; even the ones not visible ones.
3- [right-click] anywhere on the list of selected messages and selecting "Mark as unread" in the menu marks the read messages as unread.
It may take a while if you have a lot of messages in your mailbox.