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