Browsing all of your unread messages from Slack on your desktop lets you catch up on the messages you missed when you were away. While you're doing this, here are some ways you can take actions on your unread messages:
From your desktop, click All unreads at the top of your left sidebar. If you don’t see this option, click More to find it. Keep in mind that messages in muted or archived channels aren't included in this view.
From All unreads, manage how you'd like to view unread messages to help you prioritize work in Slack.
Sort unread channels
Decide how to prioritize your unread messages by changing how they’re sorted. You can sort messages alphabetically, by newest activity, or by oldest activity. On paid plans, you can sort messages scientifically— a view tailored to how you use Slack.
Mark messages as read
Click Mark as Read to clear a channel's messages when you've read them. You can also scroll to the bottom of the All unreads view and click Mark All Messages Read.
If you clicked Mark as Read too quickly, just click Undo to keep those messages in your All unreads view.
When you’d like to come back to a channel's messages later, click the arrowicon beside the channel name to collapse it. Click the icon again when you're ready to view the unread messages.
Refresh for new messages
When new messages become available while you’re viewing the All unreads view, the syncing arrows icon will appear in the header. Click the icon to reveal a fresh batch of unread messages.