I've noticed if you hit the Select All checkbox in Zimbra, if you right click on any email the "Mark Read" will be grayed out if there's even one already read email in the list, yet even if there's unread emails you can select "Mark Unread" which will mark them all unread.
This holds true even if you manually select the emails and add even one previously read email to your list. Sometimes I won't read the emails (not that critical etc) but I don't want to delete it either just incase, but I hate seeing say 157 new/unread emails in my inbox. It would be great if I could hit the "Select All" and then right click on any one email and hit 'Mark Read' and have all the messages marked as read. If it can be done with 'Mark Unread', why can't it be done with 'Mark Read'?
Also, just clicking each message to make it report as being read isn't a practical option as some nested messages may go 25 deep and only 1 within that group is "Unread" etc.
Anyhow, just a suggestion (that I really wish can be implemented)