Hi @Carrie Elliott ,
According to my test on my Outlook365, it works, you can refer to my steps to check.
- Double-click the message to open it.
- Click anywhere inside the subject line. Once you see a blinking cursor, press CTRL+A to select the entire subject line.
- Type your new subject.
- Click the Save icon in the top left corner of the message window, then close the message.
- Your new subject will show up in the Reading Pane, but the message list will still show the original conversation topic. Because Outlook shows you messages grouped by Conversation.
- In order to see the newly changed subject line in your message list, you will need to uncheck the Show as Conversation.
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