I've got a mail message with the subject "DON'T DELETE THIS MESSAGE -- FOLDER INTERNAL DATA," and the body says not to delete the message or important data will be lost. What's going on?

The POP/IMAP servers and some mail reader software (anything that uses the c-client library) use this "dummy" message to maintain state over time. Because our POP/IMAP server recognizes this message as containing metadata, it doesn't send it to POP/IMAP clients, such as Apple Mail or Thunderbird, so you'll normally never see it. But if you use an email client that doesn't recognize this as a "special" message, it will show it to you as a regular message. It's true that if you delete the message, you may lose important information, such as whether messages are read/unread, whether they've been seen at all by the client (which could lead a POP client to "duplicate" some messages), etc.