I am using the clickable program, Mail, stored in /Applications/Mail.app. There are NO visible (or invisible) mail files that I can find to click on.
When I use Mail, and display "Mailboxes" I see "In (212)" which does not display any of the 212 messages, when I select it. Instead, it says there are 0 messages.
If I choose ANY OTHER mailbox, the listing is perfectly OK and the e-mail is displayed correctly. It is as if I inadvertantly set a "DO NOT DISPLAY" switch!