Office 365 (Web) - Open a Message in a Separate Window
How to open a message in a separate window.
Open a message in a separate window
By default, you read and create messages in the reading pane, but that isn’t always what you need. Sometimes you need to be able to view a message in a separate window.
When you’re composing a new message or replying to or forwarding a message you received, you can select the pop-out window icon in the upper-right corner of the message to open it in a new window.
To open a message that you’re reading in a separate window, select the more icon and select Open in separate window.