Apple‘s email client, Mail, is pretty decent but can be bugging at some times. One of the irritating moments is when you want to mark a single message as unread in a threaded conversation. No matter what the version of Mail you are using, the issue is present is all of them.
But not many people know there is a very simple way to mark a single message as unread in a threaded conversation. It’s so simple even a second grader could do it, and never ever forget it.
All you have to do is simply click on the particular message you want to mark unread. When you do so, the message’s boundary will be marked in blue.
Next, go to the “Message” tab, follow the “Mark” option and select “As Unread”. Optionally, you can use the keyboard shortcut Shift + Command + U.
There. Now, only that single message will be marked unread.
Follow the same steps if you want to mark one message as read in a conversation of multiple unread messages.