The fact that forwarding it worked indicates that the mail folder just needs repair. Try highlighting the folder, right click, select properties and then click repair. That often works.

I can't believe it, but you're right. It worked!

What happens to make a message require repair? Don't worry, if you don't know the answer, that's okay, I'm just agog...

First, I don't understand the underlying details, but for an overview, the message folder is two components: the message file and the index file (e.g., inbox, and inbox.msf). The msf file is the index and is continually rewritten as messages are added and deleted. On occasion, this gets corrupted, causing what you saw, but can also cause the sender of one message to appear as sender of a different message -- the message is still correct and processes correctly, but can appear incorrectly on message list pane. If you seek this, it's an indication of a needed repair. There, more than you wanted to know. :)