Ever since switching to Leopard, Mail.app sometimes doesn’t update folders with new messages, even if the messages added to those folders are a result of rules in Mail.app. I searched, and found partial solutions to my liking, but here’s what I settled on.

In Script Editor, save this script somewhere:

on run
  tell application "System Events"
    if exists (application processes whose name is "Mail") then
      tell application "Mail"
        set everyIMAPAccount to every imap account
        repeat with eachIMAPAccount in everyIMAPAccount
          tell eachIMAPAccount
            synchronize with eachIMAPAccount
          end tell
        end repeat
      end tell
    end if
  end tell
end run

Next, add a new rule to Mail.app that has the condition “Any Message,” and the action “Run Applescript” using this AppleScript. All this script does is automatically run “Synchronize” on each account any time a new message is found. This ensures that all folders are kept up to date.

I’ve been using this solution for a week now, and it’s been a godsend.

Edit:

See comments for a much simpler script.