How do you open mailx to read old mail when you have no new mail and view a list of old emails?

If you type mailx at the command line when you have new mail, it nicely shows you headers of your new mail and leaves you at a ? prompt where you can do mail commands. But if you don't have new mail and you try to get into mailx, it just says "no new mail" and kicks you out. I want to get into mailx when I don't have new mail and also what I haven't figured out to do is once I'm in at the ? prompt, I want to see a list of all the headers of the already read mail still in my mailbox.

So the two things I would like to know are how to get to the ? prompt without mailing myself a dummy email, and once I'm in, how to see a list of the already read messages in my inbox.

Found it. To open without being kicked out if there's no new mail:

mailx -f

To get the header list again once in:

h