Mac mail - next message

In Mac Mail, when you open a mail message in a new window (i.e. double clicking the message), I can't figure out how to go from the current message being read to the next message (without deleting the current message). Does this feature exist, or do I have to close the current message and then double click the next message from the main mail window? I've even looked at the customization settings for the icons to place in the toolbar and there is nothing there to navigate to the next (or previous) message. Thank you for any help you can provide.

You have to close it if you open up each message in a different window. You could also install something like Letterbox to recover some screen space. That way you can have the messages in one column and the message displaying in the other, allowing you to scroll.