If you use Apple's Mail program, and you have a tendency to let your mailboxes overflow with large numbers of emails and big attachments, you may have experienced the spinning beach ball while the status window says "Reading Table of Contents." Not only is this annoying, but it often requires a force quit, and sometimes even renders your mail unusable. If that's the case, give this a try:

First quit (or force quit if neccessary) Apple Mail. Then open your hard disk, and go into "/Users/YourHomeFolder/Library/Mail." You'll see a folder with the name of your email account, like "POP-user@mail.domain.com." Open this folder, and copy the "INBOX.mbox" folder to the desktop (just in case something goes wrong).

Next, control-click on "INBOX.mbox" and select "Show Package Contents" from the contextual menu that appears. This will open a new folder, and inside it, you'll see a file called "table_of_contents." Delete this file, and launch Apple Mail, which should now work.

Once fixed, your messages may all show up as unread. To fix this, select all messages, then go "Message > Mark > As Unread."