Lost all Messages after Deleting some.

Question

Pop account using Mail v2.1 on an iBook G4 OS X (10.4.10) with 640MB Ram belonging to my wife. I'm a PC guy. Thinking of changing to Apple. :=)
My wife deleted a 'batch' (approx 50)of messages by highlighting them, clicking on Edit, and Delete. When she did that all the messages in that mailbox disappeared. She can still receive emails in that account as the New Message counter still shows when new messages arrive. She cannot see the new messages without doing a Search for them using part of the missing email address. When she does a Search the messages are there. There are 4656 messages found.
There is a small triangle next to that email account with the missing messages. When the triangle is clicked the triangle rotates, but nothing else shows. Nothing drops down below the account as I think it should. So the account is there, messages come in, the triangle rotates, the messages are not visible in that account without a Search of the Inbox.
I have attempted to Rebuild that particular mailbox, but the option of doing so is not available. 'Rebuild' is dull for lack of the proper term. She has another mailbox which does have the option to Rebuild so that is available in the program. I did attempt to Rebuild the Inbox under which the two accounts are located. That didn't do any good.
I attempted to Reindex with no change.
I ran the Disk Utility with no change.
When I look in Library, Mail, Pop(account), Inbox I see the messages she deleted. Nothing else. I can read them by double clicking on them which makes them open in Mail.
If this was a Windows program I'd reinstall it, but Mac programs are usually pretty steady.
I'm at a loss. The messages seem to be there, but not visible in the mailbox. Any suggestions?
Aloha

Answer 1

The problem is probably that the mailbox has lost the .mbox suffix in the filesystem.
Report the names of the files and folders present at the first level of the ~/Library/Mail/POP-username@mailserver/ account folder in the Finder.
In order to post the exact file names and avoid typing mistakes, proceed as follows. In the Finder, go to ~/Library/Mail/POP-username@mailserver/. With that folder open, if you’re looking at it in View > as List mode, first collapse any expanded folders so that the Finder only shows the files and folders at the first level. Now, do Edit > Select All (⌘A), then Edit > Copy (⌘C), and paste it in your reply to this post.
You’ll probably see an INBOX folder without the .mbox suffix there. If that’s the case, quit Mail and rename the folder to INBOX.mbox.
Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder. That is, ~/Library is the Library folder within the user’s home folder, i.e. /Users/username/Library.