Here is a solution from the Apple Support Article
related to this issue;
1. From the Finder's Go
menu choose Go to Folder
2. Go to this location (type or paste the tollowing, then click Go): ~/Library/Mail
3. Move the items named "MessageRules.plist
" and "MessageRules.plist.backup
" to another location, such as your desktop. Also remove the file named Envelope Index
Note: If you previously upgraded from Mac OS X 10.3 to Mac OS X 10.5, the files will be named "MessageSorting.plist
" and "MessageSorting.plist.backup
4. Next close the Mail folder and open the Preferences folder. Look for and remove the file called com.apple.mail.plist
4. Open Mail. Note: At this point you can either set your Junk Mail settings and re-create your Mail rules manually in Mail preferences, or follow the next two steps (if you re-create rules and Junk settings, do not perform the following steps)
5. Quit Mail.
6. Place the items named "MessageRules.plist" and "MessageRules.plist.backup" back in the ~/Library/Mail folder (replace any copies that were added to ~/Library/Mail when you opened Mail).
Try that and see if it helps with your issue.