Sieve filter for Return-Path header

[David Haraburda]

Hi,

I am trying to use a simple sieve filter to handle backscatter e-mails. 
  If the return path is "empty" (i.e., just <>), then I want to file it 
into a different folder:

if header :is ["Return-Path"] ["<>"] {
   fileinto "INBOX.spam.backscatter";
   stop;
}

(Eventually, I'll add another check to help catch legitimate bounces -- 
right now I'm just trying to get this to work).

I've tried using :matches and :contains, thinking that might be the 
problem.  I've also looked at the message Cyrus-imap is storing to 
verify that the header line is indeed "Return-Path: <>"

I've searched around on Google and these lists but haven't been able to 
come up with much.  Is this something that just isn't possible or am I 
missing something?

Thanks!

David
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