The ability to mark all mesages in a folder to "read"
When working as a systems engineer and utilizing outlook to store system error emails, I would utilize email folders as error email repositories. This way, you can easily see in real time which systems are failing, only one system, some systems, or all systems (OH SH*T!!).... When the issue was fixed, I would simply right-click on each folder and select "mark all as read". Wonderful feature. Now the folder shows 0 new messages, resetting it back to "normal" waiting for the next system SNAFU. Really wish you guys had this ability since marking as read page by page is a pain. Can you not simply kick off a batch job to mark all of them read? Thank you.

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justanotheruser commented
Another, simpler use case is my inbox is cluttered and I want to be able to just mark everything as read. I often only open emails I want to open and the rest sit there as read.
Similar like you can now Delete All in Spam, I want to be able to Mark All as Read in Inbox