[maildropl] mailbox doesn't exist

[<46...@ce...>]

I'have a question:
I want to do this:

if ( /^From:.*xx...@yy....*/ )
   To In.x

but I'd need to check if this mailbox exists, and if it doesn't exist, send the
mail "To In.new".

How I could do it??? 

  Thanks!
       DvD