Here's a suggestion. When viewing my mail folders (in browser), they usually run off screen as I accumulate lots of email. New email is marked at the folder level (browser) with "N" next to the folder name with new email. If I enter a folder, and think I've read all the email and leave the folder, the "N" is removed from the folder, and any new email off-screen - because the folder is viewed as a thread is now marked old ("O") and I can see I could likely skip these unless I scroll down the screen to check for any follow-up email sent to an old thread.
How about an option to mark folders with an "O" on folder (browser) view containing old email (email not marked as read -- aka forgotten/overlooked emails)? Or is this already there? Looks to me, mark old only happens in (email) index view. -- Roger http://rogerx.freeshell.org/