Fetching New Mail finished but the mail is not retrieved

Question

Mail App 2.1 on Mac OS X 10.4.8
upgraded from previous mail versions (yes)
One single POP mail account.
(17 messages in inBox, lots of messages in other folders)
Problem One:
When I click get Mail,
the Activity Viewer shows fetching new mail 8 of 11, 9, 10, 11. And then the viewer empties as if it's finished. But there is no new mail in my folders.
If I click Get Mail again, it repeats the same process.
Acts like it's successfully getting all the mail from the POP server but then no mail shows up, and I can repeat the process ad infinitum.
I have done the following without success:
removed MessageUidsAlreadyDownloaded2
Deleted my com.apple.mail.plist file and re-set up my POP account.
Neither of these tricks worked.
Problem Two:
I am no longer able to drag messages from the InBox to any of my folders.
I get the error message "The message XXX could not be moved to the Mailbox YYY"
Problem Three:
I cannot save a message as a draft. I get the error message "The message could not be saved"
All three of these problems appeared simultaneously this morning after 10am.
Can anyone help me clean up my Mail 2.1?
Thanks
mac ancient   Mac OS X (10.4.8)  

Answer 1

Verify/repair the startup disk (not just permissions), as described here:
The Repair functions of Disk Utility: what's it all about?
After having fixed all filesystem issues, if any, and making sure that there’s enough space available on the startup disk (a few GB, plus the space needed to make a backup copy of the Mail folder), try this:
1. Quit Mail if it’s running.
2. In the Finder, go to ~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong, e.g. by dragging it to the Desktop while holding the Option (Alt) key down. This is where all your mail is stored.
3. Locate Envelope Index and move it to the Trash. If you see an Envelope Index-journal file there, delete it as well.
4. Open Mail. It will tell you that your mail needs to be “imported”. Click Continue and Mail will proceed to re-create Envelope Index -- Mail says it’s “importing”, but it just re-creates the index if the mailboxes are already in Mail 2.x format.
Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder, i.e. ~/Library is the Library folder within the user’s home folder.