I was recently in Outlook in my Inbox, and did a search (default: Current Mailbox). Most results were things I'd sent, and so were marked unread. A bit later, I returned to the search results, but thought I was looking at Sent. I clicked "Mark All As Read" which I periodically do on sent mail as there is no "Mark as Read" action available to rules being applied to sent messages.
However, this had the effect of marking all the messages in my Inbox as read, including several I had marked unread because I needed to return to them.
How do I undo this?
The computer synced quickly with the server after the erroneous Mark All As Read, so interrupting prior to that happening was not practical. Folders other than the Inbox were not affected.
Best Answer
If you have another machine that regularly syncs to the same inbox, get that machine offline before opening Outlook. Then, open Outlook and do a quick search for unread messages in the Inbox folder. Mark those with a specific color Category (that you don't otherwise use) and then go back online and sync.
Then, sync with the new machine, search the Inbox for messages in that category, mark those as unread, and remove the category designation. Re-sync and you'll be good.
However, if you don't have another machine which recently synced to the server, you'll be out of luck. You'll also lose recent changes in what was marked read/unread, including the applicability to recent messages.