Junk mail filter has suddenly stopped working properly

Question

I have reset it twice, and put it in training mode. While training, it works just fine...catches the things it's supposed to and let's the safe stuff through. But when I switch it back to have it auto-filter the junk mail into the junk mailbox, it starts misidentifying everything again. Tons of junk gets through, it doesn't filter out the junk it does catch (leaves it in the inbox), and identifies things as junk that aren't - especially annoying for regular items that I receive.
Any ideas on why this has suddenly stopped? I haven't changed or adjusted any of the rules. I tried going in and changing up the identification colors just to see if that would help, but no luck.
I have three email addresses where I receive mail. Two are set up as POP accounts, and one is an IMAP account. It doesn't seem to matter which account is receiving the junk mail, and it actually gets worse with time rather than better.
Thanks for the help.

Answer 1

Are you really using Mac OS X 10.4.2, as your profile indicates?
There is probably some corruption in one or more of the files used by the junk filter. The following procedure should fix the problem:
1. Quit Mail.
2. In the Finder, go to ~/Library/Mail/.
3. Locate LSMMap2 and move it to the Trash -- deleting this file is what clicking the Reset button in Preferences > Junk Mail does. If you see an LSMMap file there, move it to the Trash too.
4. Locate DefaultCounts and move it to the Trash. This file keeps statistics about the number of messages that have been marked as junk, either automatically by the junk filter or manually by the user.
5. Locate MessageRules.plist and move it to the Desktop. This is where Mail 2.x stores all rules, including those used by the junk filter. If there is a file called MessageRules.plist.backup, move it to the Desktop too. You may also see MessageSorting.plist files there; this is where Mail 1.x stored the rules, and they are no longer used by Mail 2.x, so just move them to the Trash if you see them.
6. Look for the account folders. The name of each account folder starts with the account type (POP, IMAP, Mac), followed by the account username and the incoming mail server. Open each of the account folders, locate the Junk.mbox or Junk.imapmbox folder within it, and move it to the Trash if present.
7. Go to ~/Library/Mail/Mailboxes/. Locate any Junk.mbox folders there (their name may include the account name in parenthesis), and move them to the Trash.
Note: Messages in Junk folders with an .mbox suffix are stored locally, and will be lost forever as a result of doing steps 6 and 7. If there is a chance that you have legit messages there and you want to preserve them, move them to another mailbox in Mail first. Messages in Junk folders with an .imapmbox suffix are stored on the server and can be ignored (Mail will rebuild those mailboxes automatically).
8. Open Mail. As a result of removing the rules file, the junk filter will be disabled now. You may want to either tell Mail to go offline immediately after opening it, or shut down the Internet connection before opening Mail, to prevent it from downloading anything until the junk mail filter has been enabled again.
9. Go to Mail > Preferences > Junk Mail, enable junk filtering, and configure it however you wish.
10. Go online again if you went offline in step 8.
If the above solves the problem and you don't have any rules you'd like to preserve or recreating them is not a daunting task, just delete the MessageRules.plist files that were moved to the Desktop and be done with it.
Note: For those not familiarized with the ~/ notation, it refers to the user's home folder. You can easily locate any of the folders referred to in this post by copying the file path here, doing Go > Go to Folder in the Finder, and pasting the file path there.