highlight folders with new mails in mutt

[Philipp Schulte]

Hello,
I am trying to highlight those folders in mutt which have unread mails
in them by an "N". 
I have added "%N" to my folder-format-string and it looks like mutt
reserves space for it but it never shows up.
I have read that tools that check for new mails like biff can spoil
this setting but I removed biff and the "N" still does not show up.
Does anybody know a way to solve this?
Thanks,
Phil