The other day I wanted to clean up my mailbox (the number of unread messages had gotten quite high). Though all my recent emails were all read, there was still a large pile of old emails that I did not explicitly mark as read...
Old emails piling up...

Since most of these emails were more than a year old and not relevant at the moment, I just wanted to mark all of the messages as read. So I just right clicked on the Inbox in Outlook and pressed "Mark All as Read":
Press mark all as read

After clicking this, my expectation was that the (1) would disappear. To my surprise, this did not happen and even after switching to another folder, I would still not see the (1) disappear....

Some searching on the Internet revealed it could be due to the connection to the Exchange Server using the cached exchange mode. If I check the settings for my account, I indeed see that this is the case.

One solution to this mark all as read not doing what you expect it would do is to switch of the cached exchange mode, then perform the action, and then switch on the cached exchange mode again.

Another alternative I found is that you use the search bar to search for all unread messages and ensure you have selected "All Outlook Items" for the search area.

As with pretty much all computer problems, there are a lot of different ways to solve the problem and also for this case I found another solution :) After I selected my Inbox, I just scrolled down to the first message in my Inbox. Below this first email there will be this message:
Click here to view more

If you click on the highlighted text, your Outlook will download all messages from the exchange server, even the ones older than the limit indicated for your cached exchange mode. After all messages were downloaded, I right clicked again on the Inbox, selected Mark All messages as read and finally this (1) number disappeared!!

I think Outlook only executes the commands on the items present in the current view and then synchronizes the result with the Exchange server again. In the case of these special actions like "Mark all messages as read" I would expect the command to be sent to the Exchange server instead as the command needs to be executed on the whole folder.