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.