One of the first settings I tweak when I get a new computer – before setting up my printers, grabbing FireFox from the Software Center, or setting up Putty sessions to our hosts is to STOP Microsoft Outlook from marking a message read every time I click on it. Yes, you can add a delay – mark it read if the message has been displayed for 10 seconds. But if I’ve really read a message, I either reply to it (which marks it as read) or delete it. On the few occasions where I want to stash the message without replying, I can mark it read myself!

Teams is different than Outlook … I cannot just delete someone’s post because I’ve read it.

The channel is bolded again to indicate there are unread posts.

And the ‘last read’ marker is moved prior to the message.

Currently (13 February 2019), marking a post unread in a channel discussion does not make it show up when you filter your activity feed for unread messages. If you mark the message as unread within the Activity Feed, though, the item will appear in the “Unread Messages” filtered view.