Junk Mail only works after I disable and reenable Junk Mail Filtering

Question

Since installing Tiger, junk mail filtering does not work automatically. It neither marks incoming messages as junk nor moves them to the Junk folder.
I am able to get it to work by selecting Preferences > Junk Mail and then unchecking "Enable junk mail filtering" and then checking it. Mail then asks to "Move Junk Messages" and when I respond "Yes" it marks the appropriate messages as Junk and moves them to the Junk folder.
Unfortunately, it only works for the messages already received. New mail that is "junk" is not marked as such and appears in my inbox and I have to repeat the manual process to get it removed.
Any suggestions for a more permanent solution?
17 iMac   Mac OS X (10.4)  

Answer 1

Will it also forget the "learned" junk mail? (That took years to learn.)
No.
The MessageRules.plist file stores all Mail.app rules including the default Junk Mail rules.
All Junk Mail "learned" settings are stored in the LSMMap2 file also located at the first level within the Mail folder.
Since the LSMMap2 file can become corrupt as with any other file, this file should be included with a regular backup of the Mail folder and the Mail.app preference file at Home > Library Preferences > com.apple.mail.plist.
All Mail.app account information is stored in the Mail.app preference file.
If you experience any problems with the Junk Mail filter, your options are using the Junk Mail reset feature which resets all "learned" settings or replace the existing LSMMap2 file with a copy of a working backup copy with the Mail.app quit before doing so.
If the Mail.app preference file becomes corrupt (which I've never experienced but others have), when targeting the preference file requires re-entering all email account information. If you have a working backup copy of the com.apple.mail.plist file, you can replace the existing with the Mail.app quit before doing so without having to re-enter all email account information again.
You need to replace an existing backup copy of the Mail.app preference file only after making any changes to account information or when adding or deleting an account in Mail.
You may already have a backup strategy but if not, it is very important since it isn't a matter of if but when your hard drive has a partial or complete failure. I learned my lesson the hard way about 5 years ago. Without warning, I experienced a complete hard drive failure. I didn't maintain a backup at the time and none of the existing data was recoverable.