Description of problem:

When using exim with sieve filters, only the following subset of header tags can
be filtered on:

From
To
CC
Bcc
Sender
Resent-From
Resent-To

Version-Release number of selected component (if applicable):

exim-4.43-1.FC3.1

How reproducible:

Always

Steps to Reproduce:
1. Setup a sieve .forward file that has rules on Return-path
2. Configure exim to do filtering in the /etc/exim/exim.conf file
3. Wait for an email that should match one those rules
  
Actual results:

Default action of fileinto "inbox"; is taken

Expected results:

Rule is matched and filtered into appropriate folder

Additional info:

I know exim has it's own filtering language, but sieve filters are the only
portable solution for users.  One should be able to filter on Return-path,
X-Mailing-List, and X-Spam-Level at the very least.