Where is an "N" in subject header?

To: mutt-users@xxxxxxxx
Subject: Where is an "N" in subject header?
From: Boris Sobolev <immunogene@xxxxxxxxx>
Date: Sat, 12 Aug 2006 15:42:08 +0400
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Hello,

I just wondering why I don't have a "N" in mailboxes view for the mutt mailing 
list, 
where as all other mailboxes got marked? 
Boris.