You can have multiple messages open at one time, as described below.
Double-clicking an email message opens it in a separate window.
Opening a second message while the first is still open, displays it to the left of the first message (assuming both can fit within the browser window).
If there is not room in the browser window to display both messages, the latest one opened or selected appears on top.
When the second message opens, tabs for each open message display at the bottom of the browser window.
A new tab is added for each message opened.
Clicking a message’s associated tab will display the message.