Mozilla e-mail client and Courier-IMAP

I am using Mozilla e-mail client and the Courier IMAP server. One of the mailboxes in the lefthand panel with the mailbox hierarchy is grayed out and the name in italics. It has the correct unread messages count, but clicking on it produces no messages in the righthand panel. Other mailboxes work fine. This mailbox works fine with SquirrelMail and the same IMAP server. What does grayed out and name in italics mean? How can I access this mailbox? TIA, Jeffrey