How to view next item in Mail?

Newbie to the forum and fairly newbie to Mac...what can I say, I literally grew up in Redmond?! Anyway, here's my question:

In Mail, once a message is opened, how--from within that message--can I open the next message while keeping the original in my inbox?

Also, is it possible to 'open the next item' after deleting a message?

Thanks.