"Force" new mail in mailbox (mbox)

[Spiro Trikaliotis <trik-news@xxxxxx>]

Hello,

can anyone tell me how I can "force" mutt to think that a given mbox
folder contains new messages?

Background: On my machine, I do a nightly backup with tar. Obviously,
this breaks mutt's ability to find out if there is a new message inside
this mbox.

Because of this, I would like to set every mbox in a specific directory
(=IN/* in my case) which has a length bigger than 0 to be marked as
containing new. From my understanding, I have to update the last
modification time of this mbox.

I already tried 

  $ touch ~/Mail/IN/<blabla>
  
as well as 

  $ cat /dev/null >> ~/Mail/IN/<blabla>

but both did not succeed.

Does anyone have an idea how I can achieve this?

Thanks,
   Spiro.
   
-- 
Spiro R. Trikaliotis
http://www.trikaliotis.net/